Well, paintings need to be framed, and I think the black looks good with that piece. It's plain and does not detract from the cubism.
At least it is not a fancy gold frame (connoting "masterpiece") that all the impressionist pieces are decked in.
Those carved, gilded frames have their place, I guess, perhaps they are best on stuff from the 18th and early 19th century?. IMO, gold can be a good color for a frame, it tends to look well against a lot of different color schemes, though it's probably at its best with color schemes that are not too light. The Latins here in Miami have a taste for gaudy more than I do.
Yes, I guess you are right, the black looks good. What do you think would be best for a Monet?