Al Woodshed, author of the Ukulele Hunt website, has written an e-book on How to play Blues Ukulele. This is a great book, over 70 pages, and includes many MP3 files playing the examples from the book. Al goes into good depth covering different styles and techniques. It's the most practical book on playing blues that I've read let alone playing for the ukulele.
There are other blues material out there for the ukulele. These include Del Rey's 'Blue Uke' instructional DVD and Rigk Sauer's Learn to play Blues Ukulele. Combined with Al's e-book I think these show that the Ukulele is a great instrument for playing Blues. * Al Woodshed has an audio sample and table of contents of his book at his site. * Rigk Sauer Blues video * Del Rey Blues video