1995 Chevy S-10 Dead Battery

I kept getting a dead batter, I replaced the battery and the alternator and the battery is still getting drained. I had them both tested again today and they are both testing fine. What else could cause my battery to go dead.

Also, there is a blue wire that goes from my positive battery post up to the front of the engine compartment, can you tell me what this wire is for? It is not the ground which is a black wire attached to my negative post. This blue wire looks like it has been spliced as it has black electrical take on it. I am wondering if this wire is necessary, and/or may be causing a drain on my battery or a short?