The batteries are likely junk if they sat dead more than a month or so,the plates inside get sulphated and they wont accept a charge again...the two batteries in my Suburban were only 5 years old,but I had to scrap them,they sat too long and eventually wouldn't recharge any more--keeping them on a charger constantly can kill them too,in my experience...
I got 25 bucks cash at the scrapyard for them,used the money to buy an inspection sticker for my 82 K2500...batteries have gone way up in price,even used ones,and core charges can hit 20 bucks if you have none to trade in depending on the size now,when you buy a new one...some people claim by addingepsom salts or asprins to batteries and slow charging them for days at low amperage will "revive" them,but its never worked for me...I'd want two NEW batteries in a truck my life depended upon if I were forced to leave home!..not two that might leave you stranded in a bad "hood"...I wouldn't bother trying to revive them..
Your diesel blazer (if its a factory diesel),should have a valve near the rear spring's front hanger,thats a factory water drain valve,thats hooked up to the sending unit in the tank,there is a steel line that goes to the bottom of the tank where the lowest spot is ,that sucks out the water...you can open it and use a rubber hose to start a siphon action by blowing into the fuel filler neck at the gas cap and any water that has accumulated at the bottom of the tank will be removed as the fuel is siphoned out--you can see the difference in the fuels color once all the water is out--do this on level ground...
(you might not have any water in it too though,like I said about my pickup,despite it sitting long periods and not having much fuel in the tanks,I never saw any evidence of any in my tanks--except maybe the leaky one that may have rusted from the inside out from water!)...water is heavier than diesel and will settle to the bottom of the tank eventually,it needs to sit undisturbed awhile if its been sloshed around...