I have a 2003 S10 Blazer. Cranks and runs good till I reach around 3000 rpm. Then the check engine light starts flashing and it starts misfiring . Does anyone have an idea what the problem might be?

 

Flashing check engine light means catylist damaging misfire- often from raw fuel or coolant getting into the exhaust.

Scan for codes, and let us know what you come up with.

Check your oil for coolant( it's less likely to be this, but easy enough to do a quick check)

If nothing found there, do a fuel pressure leakdown test to see if you may have a leaky injector. Follow the guidelines in this thread to do so:
Fuel pressure testing