I have a 1984 s 10 blazer and it seems like there's a vent open near my feet, but I don't know how to close it. please help thx, swissmiss

Everybody has an issue with this. To stop it you need a shut off valve of some sort to the heater core. The heat generated from the hot water flowing through it just caries through the whole floorboard.

So is there a shut off valve or do I need to install one? If I need to install one, where can I get one, and will shutting off the heater core make the engine over-heat?

No you don't have one.
You can find one in a junk yard or parts store.
No, cutting the water off to the heater core will not cause overheating. at least I haven't heard of it yet. I have done many cars like this. In fact a lot of honda's and mazda's come with the shut off valve.

I was surprised that our '99 blazer does not have heater valve. So, heat is ALWAYS in the dash. Depending up the baffle settings, the air will pass through the heater core or not. If this problem was occuring on this vehicle then there woud be a problem with the baffle being stuck in the heat position. The baffles are controled by vaccum hoses. You might want to see if any hoses are cracked or fallen the accuaters

You can obviously by-pass the heater core to get the heat out but this is just a band-aid fix and does not solve the problem.