I have done EXACTLY the same, and the greasy blackness is awful and
comes from the rubber. These tumblers are OK for stones, but with metal
and detergent the black greasiness results.
So, a solution: 1) The greasiness comes off with concentrated ammonia,
soap, warm water and a soft brush. But loads of work.
- Buy a proper hard-plastic barrel, or improvise. I have used a length
of PVC drainpipe and special caps which are sold at plumbing suppliers.
Tumble the steel shot with a mixture of neat washing up liquid detergent
and coca-cola (yup, I am serious). Put it in the barrel and whizz it
about with a spoon until the fizz goes out of the coke, then barrel it for
about 2 hours. Clean out the barrel and put the pieces in with the clean
shot, liquid detergent and some ammonia, but not too concentrated. If you
like, add some fine sawdust as well, to help polish the gunk away.
I hope that this helps.