Black-tar is, as the name implies, a black (or very dark brown) compound, generally pretty sticky (though it can be dried into a powder). White powder heroin is, as the name implies, a white powder.

Fentanyl is a white or yellowish powder.

For a variety of reasons, it is far more common for white powder heroin to be laced with fentanyl (which is hundreds of times more potent).

"Powder" Heroin is most often light-ish brown/tan (the darker the better, generally -- with exceptions of course). It will still cook up to the familiar color of motor oil. White powder is almost unheard of these days. While generally considered to be of better quality, it's often referred to as "China White" -- I believe this stems from the heroin boom of the 1970's when most of it was imported from the Far East after the Vietnam War era.

What is sold as "china white" nowadays will often contain fentanyl

Indeed. That's mostly what we're seeing here in central New York.

In addition to what the others have said, black tar is essentially heroin that is made by skipping a few of the steps in the "standard" heroin synthesis process. Whichever cartel controls West Coast distribution decided that was good enough for them, and customers would buy it regardless. I'm actually curious as to why the other cartels bother producing powder heroin at all.

Presumably because it packs to a higher density and is purer, and since sentencing is done by weight irrespective of how much active drug it actually contains, it winds up being essentially a forcing function toward making the strongest substance you can. Such laws therefore are almost entirely responsible for why drugs have gotten stronger and stronger over the years, with the increased risk this brings.