So my girlfriend turned right on a red light and when she went to accelerate there was about a 2 second delay/lag where the gas didn't give the car any power, so the car behind her almost hit her. It's a 1998 blazer.

Did she probably just not hit the gas hard enough or could this be an actual problem? What can cause this?

Lots of things.

Clogged fuel filters, low fuel in tank, tpms, maf etc etc.

Try to replicate the problem a few times and bring that information back to us.

I had this issue with my jeep. I always had it below half tank of gas, most likely the issue..