I remember it was a demon movie, a girl walks alone through a park at night and this demon guy throws or commands this black, formless substance into her mouth. She swallows it and it does something evil to her, I guess?
Burned at the Stake
What you're recalling is the R rated version, which for some reason isn't on video.
I've been looking for this one myself for years, but all I keep finding is the "edited for television" version.
On a side note, in my quest to find the R rated version of Burned at the Stake ( http://www.imdb.com/title/tt0082116/ ), I found out that the little girl who played Dorcas Goode (Jennine Babo) recently did some really cheesy homemade sci-fi for Ikuzo Studios, with production values not too far off from the works of The Polonia Brothers, with the exception that they're nowhere nearly as painful to watch.