Took car to mechanic said that something keeps running after you shut your 95 Chevy blazer off and kills the battery what could it be?

A common problem is the alternator. If one or more of the diodes are bad it can cause a power drain. You can have your alternator tested at many of the auto parts retailers, they'll do it for free especially if they think you might buy the replacment parts from them. If that isn't it, it could be that the accessory contacts on your ignition switch have shorted and are remaining on after you turn off the key. If that's the case you'll need to replace the contacts. Last option is the possibility that someone has added after-market devices that run straight to the battery or other circuit that is energized constantly. On the other hand it could also be that your mechanic is clueless and the real problem is in the battery. When some batteries get older the plates start to drop pieces of lead sulphide. If that material builds up in the bottom of the cell sufficiently to make contact with both a positive and a negative plate, it will cause the cell to discharge rapidly. If you don't test the battery properly you can't know that a cell is shorted. When they test the charging system they also test the battery for your, so that solves your problem. Whichever it is, I'm quite surprised that the "mechanic" didn't bother to tell you which "something" is running. Isn't that HIS job?