I usually do my blocking for 1 hour at RT using 5% skim milk, but now I am doing it already for 2 hours or so because the background is not clean with just an hour blocking. My skim milk powder is somewhat old, we bought it last 2011. Is it possible to increase the concentration of the powder or should I just buy a new one.
I would not go higher on your blocking, but would instead go longer on your washes. Generally speaking, washing can have as much an effect on background than blocking or binding. Go for minimum 3x 10 minutes in TBST for washes, 20 or 30 minutes if you still have high background.
5% non-fat dry milk in TBST is pretty safe for most antibodies though. You could, alternatively, try 5% BSA or fish gelatin, but with obvious cost increases.
Besides what Dr. Yaron has recomended you, maybe you'd want to try using a biotinilated secondary antibody. It would add the additional steps of using an streptavidin-HRP complex (I used to use the AB-complex from Vectastain), but it should reduce background: it worked for us with some very annoying anti-goat antibodies. Besides, relatively new/fresh skimmed milk wouldn't hurt either.
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