So I currently have the stock ZR2 rims on my 4door 4x4 1997 chevy blazer.
I want to get some 15x8 -19 offset 3.75 backspace rims with some 31x10.5x15 tires, Ill do some fender cutting, think it will fit?
Understanding all the tire numbers has been really hard for me and trust me I have been trying lol.

 

I strongly suspect you will require some major fender trimming unless your truck is also lifted, and maybe even then. With 3.75" backspacing on an 8" rim, and a 10.5" wide tire, the outside edge of your tire is going to be in the neighbourhood of 2.75-3" further out from where stock 235/70/15's sit, and a good 2" taller. You will definitely rub on the back of the fender well when turning, and possibly on the front of the fender as well. I'm guessing at stock ride height you'll need to remove at least 2" of material from the back of the fender to not rub when turning.

In my experience, with 31x10.8" (265/65/17) tires on 17x8" rims with 5" of backspacing and 4" of lift (2 BL, 2 suspension), I still needed to trim my fenders about 2" to clear with suspension travel when turning.