Here are options i would consider from easiest to hardest. If your worried about mileage get a diesel or drive something else.
1. New 350tbi w/vortec heads
2. 383tbi w/vortec heads -my personal favorite
3. 454tbi -not something I would do
4. 6.2 with or without a turbo kit (circle "with")
5. 6.5? I really dont know mucha bout this swap.
6. A newer, non genIII/IV multiport motor (vortec 350/ lt1)
7. genIII/IV swap
8. cummins swap
9. jet engine - have fun trying to find t-case adapters.
There are two main points that you need to think about
1. You have tbi. switching to something else, will be time/money consuming
2. get what you like the most, you will regret it in the future.
I know of both of these and have suffered twice, and Im just now getting what I want.
Edit: You drive a blazer, paying for gas sucks, deal with it. Trying to save gas everywhere will only cause headaches.
Edit #2: None of the above options reflect how "badass" a motor might be.