Doesn't Washington DC have a representative from another state?

No, Washington DC isn't part of a state. They have a delegate in Confress of their own but that delegate cannot vote

They have a representative in the House of Representatives who can participate in committees and other discussions, but who cannot vote on the floor of the House (where it really matters).

No, it doesn't.  It does have a nonvoting representative of its own, who is from DC and is not from any state.

The ridiculous movement to make Washington DC a state is an attempt to add two Black Senators and a Black Representative to Congress.

Better would be to make DC like a US territory for taxation purposes. US territories don't have voting representatives in Congress because they don't pay federal income taxes.  

From another state? I don't know what that means. Residents of D.C. have no voting representative in the House or the Senate. That's why there's discussion of statehood. It would give those tax payers representation. 

I'll assume you mean a representative in Congress.  The answer is no.  Like Puerto Rico, they have a non-voting delegate who can sit on committees but has not voting power in the end.

Not that I know of.