Black tar heroin is a pretty impure form of heroin, they skip a few steps, not sure which ones exactly, never looked much into how heroin is manufactured. Tar usually is pretty bad, but there is some really good tar out there. The really good tar tends to cook up really light brown/yellowish. Overall, I like powdered a lot more; I get a way better rush from it. Unfortunately powder is pretty hard to find around here(California) and when it is around it seems to come and go. Some people like tar though because it has a high level of monoacetylmorphine compared to powdered, so it lasts longer. But the rush just isn't as good in my honest opinion. High quality tar is around the same level of average powder.
Its a good idea to double filter with tar if you aren't able to get wheel filters, even if it cooks up really light brown/yellowish, it can still be kind of gummy. I wish the Mexicans would start producing powder...but its kind of the evolution of heroin. 50 years ago, the east coast supposedly had mostly black tar heroin. Hopefully we will start seeing more powder in 10-15 years on the west coast.