96 blazer I have 2 relays approx 3-4 inches below the radio which I assume are for the turn signal and hazard lights. My question is right above that is another slot and it was empty. Am I missing something there? (Picture for reference)

 

I don't know specifically what plugs into that port, but its pretty common for there to be unused connectors. Typically they're for features on different models that your truck does not have. GM made universal harnesses for all models of these trucks, and then just left unused connectors hidden away.

For example, my truck came with manual cloth seats, but has the connector under the carpet that allowed me to bolt in heated memory leather seats and they just plugged right in. No additional wiring necessary.