Anyone use 235 70 r 15 to replace 31 10.5s read they were virtually the same

 

Not really.
31 10.5 would turn out to be 265/75's

The diameter is bigger. It's around 15 % more. The 235/70 may be the OEM size.
Bigger tires without adjustment of your ODO will make you go faster than the indicated speed.

 

Here is a link to a tire size calculator, should help you determine what size you want.

https://www.tacomaworld.com/tirecalc

 

Your probably right typed the
numbers wrong just wondering if anyone has made this replacement before and how well it worked

 

It looks like your blazer may have a lift?
On my ZR2 blazer, my old 31x10.5'sx15 were an off brand sold by walmart, they were actually 29.5 inches. I did a lot of research to get the biggest tire without turning issues and went with the B.F. Goodrich All-Terrain KO's (31X10.5X15) as they have a bigger circumference that most other manufacturers.
Check with the manufacturers website and you'll be surprised that just because they say 31", actually some are around 29.5 inches.

These KO's are awesome.

 

using the tire calculator, 255 65 R15 say they're roughly 10 in wide. Without adding much height at all over the 235 70 R15, will these fit the 4 by 4 second gen Rims without rubbing inside? Wanting wider but not necessarily taller on my 99 Bravada. current 235 70 R15 Wranglers are getting a little thin so their circumference is already smaller than new. Any thoughts ladies and gents? Otherwise I was going to go with the 30 by 9.5s but those seem more expensive.

 

Think error was right gonna go with 265 or 275 75s if I can't find any 31s