Here are more great tracks you can get for practicing blues.
These are good simple practice tracks for play along to. If you have used my free ones and are looking for more, you may want to check out these practice tracks. They cover a variety of styles, but Musicians Friend also has more to choose from, in case you want blues only tracks.
Slow down tunes for learning
Here is a tool I recommend for slowing down music, so you can easier figure out what is being played. It's called Song Surgeon, and it's got some nice features. Check it out.
I have put together some blues backing tracks that you can use to practice with. They are mostly just simple blues backing tracks. I only have a few so far, but I am working creating more. Some of these I did not create myself. If you use one of these backing tracks and record something with it, let me know - it would be fun for me to hear you play over them! For commercial use, you must get my written permission for the songs I wrote.
I recorded some of these tracks with Cubase SX2, using my Vox AD50VT and my Squier '51 or Squier Tele. Nowadays, I use GarageBand and Ez Drummer for my recording. The sounds were created using Line 6 Toneport and Pod Farm, or else miking any of my guitar amps. Some compression and delay were added after recording.