Have a 2000 chevy blazer overnight something kills battery

Yes you need it,it controls lighting and such.What you were told,and what is tested,and diagnosed,is another thing,so ,do this,Unplug the alternator plug in on the alternator,and see if it goes down during the night,if it does not,replace the alternator.The regulator in the alternator could be open.Or a diode in the alternator could be blown.Have the alternator checked on the vehicle at the battery also,the auto parts store will do this for free.

Well a fuse is just a safe guard, it does not draw a charge. You have a parasitic load. Something is either plugged in like a GPS, MP3 player or (may sound silly but have seen it) the glove box light is not going out. Look for something like these. OR, you have a bad connection somewhere that is cause a small draw and draining your battery.