Thanks for the chance to help. I am an attorney with over 12 years experience. Hopefully I can help you with your legal question.
I am sorry for this dilemma.
It is illegal to discriminate based on race. It is not illegal to discriminate based on suspected drug use.
So if they are targeting you based on your race? That would be an example of illegal employment discrimination...and you could file a complaint with the EEOC (equal employment opportunity commission) to stop the discrimination.
But if they are doing this because they think you are doing drugs, even if they are wrong, and not based on your race? That is not against the law.
What you describe is some evidence. The fact you are the only african american at the company is some evidence.
But understand what your embarking on if you step into this.
The defense may be that "they suspect you of drug use...that you are not performing well on the job".
If you launch into this, the employer can defend by showing (or trying to show) your faults. Not fun.
Still, if you want to win, that is "part of the game".
And if you do file a claim, your able to go through the "discovery process" so you can get access to employer information (like emails, records, etc) that may give you more evidence
That would be the key here...if after you file the complaint, you get more evidence via emails and documents...that is how these cases are won.
You can start the complaint process here http://www.eeoc.gov/employees/charge.cfm
Let me know if you have more questions, happy to help if I can