Blockade is the immobilization of the enemy by means of controling squares of a certain color, it most often refers to the blockade of a passed pawn by a piece (diagram below). The knight is a particularly strong blockader since it can jump over other pieces and is not restricted in its mobility while performing blockading duties.
Whites passed e-pawn isnt going anywhere due to the blockading knight. The knight on e6 also places pressure on the white f-pawn and the d4 square.
The following game shows the strength of a blockading knight in the middle game and the superiority of the knight over a bishop as a blockader.
1. e4 e5 2. Nf3 d6 3. d4 Nf6 4. Nc3 exd4 5. Nxd4 Be7 6. Be2 0-0 7. 0-0 Nc6 8. Nxc6 bxc6 9. b3 d5 playable here would have been ...Re8 and ...Bf8 with a restraining strategy 10. e5 Ne8 11. f4 f5 otherwise white advances f5 with a strong attack 12. Be3 g6! 13. Na4! Ng7 14. Qd2 Qd7 in order to follow up with ...Rd8 as soon as possible 15. Qa5 Ne6 the black steed takes up his blockading position 16. Rad1 Rd8 17. Nc5? A positional mistake white should seek to keep the knight as a potential blockader. The situation is this: the two knights are the chief actors here (because they are the most effective blockaders) and whoever gives up his proud horsemen for a bishop gets the worst of it. 17...Bxc5 18. Bxc5 Bb7 19. Rf3 Kf7 20. Rh3 Kg7 21. Rf1 Re8 22. Rhf3 Rad8 The a-pawn is poisoned. If 23. Qxa7? Ra8 24. Qxb7 Reb8 There is little that white can undertake. 23. Rd1 a6 24. b4 Kh8 25. Qa3 Rg8 26. Qc3 Rg7 27. Kh1 Rdg8 black plans ...g5 and in this the blockading knight at e6 would render priceless service 28. Be3 c5! the pawns lust to expand makes its presence felt the diagonal is opened up for the black b7-bishop 29. Rg3 (better is 29. bxc5 d4 30. Rxd4 Nxd4 31. Bxd4 Bxf3 32. Bxf3 with two bishops and two pawns in exchange for two rooks) 29...d4 30. Qa3 g5 31. Bc4 gxf4 (31...Bd5 would have been good if only to preserve the knight) 32. Bxe6 Bxg2+! 33. Kg1 (33. Kxg2 [33. Rxg2? Qc6] 33...Qc6+ 34. Kf1 fxg3 35. Bxh8 gxh2 is bloody -+) 33...Qxe6 34. Bxf4 Bb7 35. bxc5 Qd5 36. c6 Bxc6 37. Kf2 Rxg3 38. hxg3 Qg2+ 39. Ke1 Bf3 40. Qxa6 Qg1+ 0-1 Leonhardt-Nimzovich, San Sebastian 1912
In the next game black is forced to stop whites attack but in doing so allows him a passed pawn, he then blockades it and eventually obtains the better game.
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. d4 exd4 6. O-O Be7 7. e5 Ne4 8. Nxd4 O-O 9. Re1 Nc5 10. Bxc6 dxc6 11. Nc3 In this game we have a battle between the black bishop pair against whites superior pawn structure. Black's Queenside majority is crippled while white has a candidate passed pawn on e5. White also threatens the f4-f5 advance which would give him good chances for a kingside attack. 11...f5! this stops whites attacking chances but gives white a passed pawn, this pawn however can be blockaded from e6. 12. Nce2 to challenge the blockade 12...Ne6 13. Nxe6 Qxd1 14. Rxd1 Bxe6 15. Nf4 (On 15. Nd4 black can play 15...Kf7 16. Nxe6 Kxe6 when the king is an ideally centralized blockader) 15...Rad8 16. Be3 Bc8 17. Nd3 b6 Now black is ready to play ...Be6, ...c5, ...g5 and general expansion supported by his bishops. This would be very depressing to the first player so he tries to prevent ...c5 18. b4 f4! activating his bishops 19. Nxf4 Bxb4 20. Ne2 Bf5 now the bishops mop up 21. c3 Ba5 22. Rac1 c5 23. f3 Be6 24. Kf2 Bc4 25. Nf4 Rfe8 26. Rxd8 Rxd8 27. a3 Re8 28. e6 Bxe6 29. Nxe6 Rxe6 30. c4 Kf7 31. Bf4 b5 32. Be3 Bb6 33. cxb5 axb5 34. Rb1 c4 35. Bxb6 Rxb6 36. Ke3 c5 37. Ke4 Ke6 0-1 Vesely-Pachman, Prague 1951
Here is a Nimzovich game from "My System" where he shows the crimping effect a blockading knight can have on the enemy's position.
1. e4 e6 2. d4 d5 3. e5 c5 4. Nf3 cxd4 5. Nxd4 Nc6 6. Nxc6 bxc6 7. Bd3 Qc7 8. Bf4 g5 not quite sound but leading to interestng play 9. Bg3 Bg7 10. Qe2 Ne7 11. O-O h5 12. h3 Nf5 13. Bh2 (If 13. Bxf5 exf5 14. e6 f4 15. exf7+ Kxf7 and black is better) 13...g4 14. Re1 (the answer to 14. hxg4 would be 14...hxg4 15. Qxg4 Rxh2! 16. Kxh2 Bxe5+ followed by ...Bxb2) 14...Kf8 15. Nc3 Qe7 16. Bxf5 exf5 17. Qe3 Rh6 18. Ne2 c5 19. Nf4! this knight is to be regarded primarily as the blockader of f5 and its adherent mass of pawns; but in addition he acts as an "anti-blockader" for his pawn at e5. Notice the blockading f4 knight is strongly supported by whites Bh2 and hinders the activity of black Bg7 and his Rh6 19...d4 20. Qd3 Qd7 21. Qc4 Qc6 22. hxg4! the necessary prelude to Nd3 (If at once 23. Nd3 gxh3 23. Qxc5+ Qxc5 24. Nxc5 Rg6 25. g3 and white stands badly) 22...Ba6 23. Qd5! Qxd5 more interesting would have been 23...hxg4 with the resultant march of the e-pawn, for instance 24. e6 with an attack on the Queen 24...Qxd5 25. e7+ Ke8 26 Nxd5 followed by check at c7. 24. Nxd5 Bc4 25. Nf6 hxg4 26. Bf4 Rg6 27. Nd7+ winning the c-pawn and with it the game 27...Ke7 28. Nxc5 Rc8 29. b4 Bh6 30. Rad1 Bxf4 31. Rxd4 Rh6 32. Rxc4 Rch8 33. Kf1 Rh1+ 34. Ke2 Rxe1+ 35. Kxe1 Bxe5 36. Nd3 Bd6 37. a4 a5 38. b5 Rh1+ 39. Ke2 Rh2 40. Nf4 Bxf4 41. Rxf4 Rxg2 42. c4 Rg1 43. Rxf5 Ke6 44. Rd5 Rb1 45. Rd8 Ke7 46. Ra8 Rb4 47. c5 Rxa4 48. b6 Rb4 49. c6 Rxb6 50. c7 1-0 Nimzovich-Von Freiman, Vilnius, 1912
In the next example white achieves a superior position that looks crushing but black by means of a suprising exchange sacrifice manages to set up a blockade and hold the game.
1. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. e3 O-O 5. Bd3 d5 6. Nf3 c5 7. O-O Nc6 8. a3 Bxc3 9. bxc3 b6?! (better is 9...dxc4 10. Bxc4 Qc7 the point being that the knights are better positioned to influence events on key parts of the board if the pawn structure is not subject to any sudden change) 10. cxd5 exd5 11. Bb2 c4 12. Bc2 Bg4 13. Qe1! black may now double whites pawns by 13...Bxf3 14. gxf3 but whites kingside would be very solid and he would have an extra pawn in the center, as well as chances of play on the g-file 13...Ne4 14. Nd2 Nxd2 15. Qxd2 Bh5 black wants to challenge bishops on the b1-h7 diagnonal 16. f3 Bg6 17. e4 Qd7 18. Rae1 dxe4 19. fxe4 Rfe8 20. Qf4 b5 21. Bd1 Re7 22. Bg4 Qe8 black is keeping e5 well covered if he allows both d5 and e5 he will be squashed 23. e5 a5 24. Re3 Rd8white has a massive positional advantage 25. Rfe1?! inaccurate Bronstein suggested 25. h4!? 25..Re6!! Petrosians idea here is to create an unbreakable blockade on the light-squares by maneuvering a knight to d5 26. a4 Ne7 27. Bxe6 fxe6 28. Qf1 Nd5 the point 29. Rf3 Bd3 30. Rxd3 cxd3 31. Qxd3 b4 with this the draw is secured since the knight on d5 is so firmly entrenched on d5 the white bishop is ineffective 32. cxb4 axb4 33. a5 Ra8 34. Ra1 Qc6 35. Bc1 Qc7 36. a6 Qb6 37. Bd2 b3 38. Qc4 h6 39. h3 b2 40. Rb1 Kh8?! (40...Qxa6! and black is better) 41. Be1 1/2-1/2 Reshevsky-Petrosian, Zurich 1953
The next game is a short "Grandmaster Draw" but black utilizes a blockading system to achieve equality.
1. e4 e5 2. Nf3 Nc6 3. Bb5 Nf6 The Berlin Defence Kramnik succesfully used it in his match against Kasparov. 4. O-O Nxe4 5. d4 Nd6 6. Bxc6 dxc6 7. dxe5 Nf5 8. Qxd8+ Kxd8 the key starting position of the defence. As you can see, while white has a King side majority, Black has the Bishop pair with which he would try to balance the position and prevent white from advancing his pawns. 9. Nc3 Ke8 10. h3 Be7 11. Bg5 Bxg5 12. Nxg5 h6 13. Nge4 b6 14. Rfd1 Ke7 15. Kh2 h5 Black doesn't have the Bishop pair but has put a Blockade on the Light squares 16. Ne2 Re8! Black now starts to point out at white's weakness, the e5 pawn itself and at the same time improves his King position by way of an artificially castling. 17. Nf4 An attempt to strengthen the e5 pawn with 17. f4 allows 17... Ne3 This is a typical example of the problems that white faces in the Berlin. Though white can gain an advantage mainly through the advance of his pawn majority, he has to be careful not to conceed good squares for the Black pieces. 17... Kf8 18. Nxh5 Rxe5 19. Nhg3 Be6 20. Rd2 (After 20. Rd2 Nxg3 21. Nxg3 White still has a pawn majority on the King side but Black's Bishop which has come alive is a much better piece than the Knight on g3. 21... Rd5 22. Re2 (Ofcourse 22. Rxd5 allows Black to undouble his pawns 22... cxd5) 22... Rad8 and Black is very much comfortable) 1/2-1/2 Anand-Kramnik Advanced Chess Match gm 6, Leon, Spain 2002