ok, so while we left pickups passive, there is no problem with scratchy blend pot. but this annoying noise is there in both cases, even with everything grounded, what i've tried. when the trebble pot is lowered to minimum, the noise is gone too. so the last thing that comes to my mind is to make the guitar, for this purpose, passive, so bypass whole electronics and battery circuit. i'm pretty courious if it will be humming even with this diagram too. if yes, then it's pickups where is the problem, if not, the electronics is noisy, so change will be the best way..
edit: yesterday i tried to bypass the electronics and my bass remained deadly quiet after that. So the verdict sounds that we've got s shity electronics in there. im gonna order some new parts and completely rebuild my t-bird to passive, i didnt like so much trebble and bass after some time anyway. what i want is to make 1x volume pot, 1x balance pot and 2x tone pot, for each pickup separatelly. there is option to buy one push/pull pot and make it switchable between power supplied and non-power supplied pickups. so this is my next job as soon as my next sallary will come :P