San Antonio is a usually a great blues town, but it looks like slim pickings this weekend.
Here's what I found:
Casbeers at the Church (
http://www.casbeers.com/): Bill Kirchen, Saturday @ 9 PM
Sam's Burger Joint (
http://www.samsburgerjoint.com/): Lincoln Durham, Saturday, 6:30-8 PM
Nightrocker Live (
http://nightrockerlive.ning.com/): Boxcar Satan, Coma in Algiers & Blackwater Revival, Saturday @ 10 PM
I would recommend you get on the mailing list for the San Antonio Blues Society (
http://www.sanantonioblues.com/) so you can get show info sent to you every week. Both Sam's Burger Joint and Casbeers are great little venues. I've never been to Nightrocker, so I can't vouch for it.
I hope this helps. Have fun!