The oil cooler lines leak where the rubber hose is crimped to the metal pipe. Can't be tightened. Common Blazer problem, and it seems no matter what brand replacement you buy they will eventually start leaking. If you search the forums there are threads by people who have come up with their own permanent fixes to the problem.

Might want to check ASAP if the leak is coming from the oil lines, the hose will eventually pop off the fitting which will leave you on the side of the road. Happened to me on my first Blazer, didn't know about all the Blazer quirks back then.