Very Worn Roman Coin Identification

I think this is Vespasian - I reckon I can just about make out VESP[ASIAN] behind the head. The portrait is right for him, but as Jskirwin has said, the portrait for Titus is quite similar too.

The figure on the reverse is less clear, but looks to be holding a cornucopia and standing left. There are a few goddesses who it could be. If they are holding something in their right hand, that would narrow it down. The coin is probably something like this:

The black is probably patina - the metal showing through means you've either overcleaned it (unlikely, seems you know what you're doing!) or the patina was no good to begin with. I doubt there's anything more to be done with it that would improve it. A good way to show up some details hidden in the dark patina is to press the coin into some foil - especially thin foil, like you get with a kitkat - the contrast is better that way.

The bust appears to have traces of a radiate crown.

The radiate crown would indicate that this is a dupondius (as opposed to the smaller denomination 'As' or larger denomination sestertius).