Wider tires?
 

Has anybody mounted 255 70 15s on a stock Blazer 4x4 rim? Will it fit?

You should be good to go

I ran 255/70/15 on the stock 4x4 rims for my Blazer for 3 years. No problems at all, and they look great.

Great. I think I found a good deal on some 255's... Was looking at 30's but man they are too expensive and not much bigger.

255/70/15's are a touch shorter, but they are wider. I like the look of wider tires.