Because of the design of the Blazer... 31's are all you'll be able to fit with 2 inches of lift...
There's 2 directions you can take...
A 2 inch body lift which I'd recommend if you're the ocassional offraoder and do alot of miles on road.
Crank the Torsion Bars up 1.5 to 2" and get 2 inch shackles for the rear leaf springs to replace the stock ones. To crank the torsion bars, tighten the bolts at the end of them under neat the truck, right by the tail-piece of the Transmission, you'll see them on the cross member.
**** I wouldn't recommend this for a daily driver though as it will can and usually will cause premature wear of your CV joints.**** The only thing is, it's cheaper than a Body Lift, so if you want to run the risk, then go ahead.
If you find that you're rubbing your fenders a bit while you're wheeling real hard, trim the metal off, and don't be afraid because you'll just damage the fenders anyway, but you shouldn't rub if you use stock offset and backspacing if you get aftermarket rims... Essentially shoot for 6 inches of backspacing on any rim you buy, and you'd be best to go with 7 inch wide rims, 8's should be alright also.