I just purchased a 2001 Blazer in Gilroy California , test drove it,it was fine, and a week later picked it up and it over heated 5 minutes into my drive home.I checked the coolant levels and it was empty,filled the cooling system up again ,and made it home. The next day it overheated again.I feel that the overheating condition was pre-existing and the selling party didn't disclose this.What is my legal recourse? I have had it for 2 days and don't want to keep it if at all possible.

Generally a private party sale is As Is. If you believe the seller hid a problem (didn't you have it checked by your mechanic?) you can try suing for the cost of repairs.

What is the problem, it may be as simple as a bad radiator hose.

But with any used car sale, it is the buyers responsibility to have it checked out by an independent mechanic.

Since you don't seem to know the problem, if it is losing fluid, there is a leak somewhere and normally this is a minor issue. ( hose, water pump, bad radiator cap.) So first find out what is wrong, since you can't claim they would have know about it, unless you find the problem first.

Next what does the paperwork say, was there a warranty or was the sale "as is"

Even if the seller is liable, they may only be liable to the cost for the repair.