I have a 99 chevy blazer 4dr 4x4 base model, I want to upgrade the wheels & tires.
I want to up grade to 15x8? & the widest, basically biggest offroad tire I can, what sizes of tires would work without body modification (trimming,lift)

 

It depends on the wheel offset. The further the tire sticks out, the smaller the tire you can fit. Stock wheels can fit a 31" tires with some rubbing against the frame and rear of the front fender. A 15x8 wheel with a RWD offset will reduce your tire size to nearly stock.

 

Like said above, you never listed the backspacing of the wheel. But with no modification to the body and no lift, the biggest tire you can run SAFELY is a 30/9.5/--. (the -- indicates whatever size rim you are running) Anyone who says different is crazy. Any rubbing on the frame or body is asking for trouble. You could destroy your tire or worse, flip your blazer if the tire comes in contact with the frame.

 

I agree. Your rig can accommodate 30x9.5x15 tires with no modifications. If I were you I would put some lift so I can have a more aggressive set of tires for off roading.

 

31x10.50 will rub with no lift so yeah 30s