What does it mean when you dream your drowning in black oil?

Drowning by itself can mean you are feeling overwhelmed by emotions and issues. Perhaps something that you repressed may be coming back to suffocate you.

Now you said "black oil", rather than any other type of refined oil so I shall take it to mean crude oil.

Oil, according to popular websites, can signify dreams of great riches, or perhaps a reminder to increase your social activity. Another definition could be you need to be more environmentally conscientious.

In summary, You are feeling overwhelmed by certain aspects of your life, which are inhibiting you from engaging in social activities, becoming more financially stable or are forcing you to be environmentally unfriendly.

To me this dream seems to mean you are being suppressed, sucked in or smothered by something. The black oil (and I assume it is a very "heavy" substance") could mean that what you are being smothered with is extremely hard to get out of.

I do realise this is a very vague answer, but it really depends on other details in the same dream. A single aspect of a dream doesn't give you its meaning. Its like if you read a single chapter of a book half way through, you'd get nearly nothing from it. I suggest you use this information to assist you in your interpretation, but by no means use this as a guide.