Blackwork is the modern term for what the Elizabethans called "Spanish Stitche", among other names. It was not always black thread. The Elizabethans used red, brown, green and blue as well.
The Elizabethan Practical Companion
M'Lady Osanna Blackwork Designs
Blackwork Embroidery Archives
Embroidering a Blackwork Coif
16th Century Blackwork Samplers
Blackwork Techniques and History
Fill-in Patterns for Blackwork Embroidery
Skinner Sisters - What is Blackwork?
Blackwork Designs from 16th century sources Details of portraits with the blackwork pattern charted
ANG: Blackwork - An Introduction
Precision Blackwork Techniques by Leon Conrad
Historic Needlecrafts Some freebie Blackwork Patterns and SCA Kingdom cross stitch kits for sale.
Online Blackwork Merchants
Elegant Detail Blackwork