[EricH]

From Update #9:

<R12>The color of the BUMPERS will be used to identify the ALLIANCE to which the ROBOT has been assigned, red or blue. Therefore, each ROBOT must be able to display red BUMPERS and blue BUMPERS. This may be done via either of two acceptable methods:

B. Alternately, the ROBOT may use changeable BUMPER covers. The BUMPER covers
− may be removable, reversible, or fixed
− must completely enclose the BUMPERS
− must show only a single color such that when the BUMPER covers are in use, only fabric of the assigned ALLIANCE color may be visible.
− must be constructed solely of fabric and a fastening/restraining system to hold the cover in place. The fastening/restraining system must extend no further than one inch beyond the FRAME PERIMETER (i.e. no further than any other hard parts of the BUMPER - see Rule <R07-N>). Please note that the fastening/restraining system MUST be designed with robust performance in mind. The restraints must hold the cover in place during vigorous interactions with other ROBOTS and FIELD elements during the MATCH without allowing the cover to come off.

Also see <R07-F>.

Based on your description, I'd say questionable or illegal. If any black shows, illegal under <R07-F> and/or <R12-B>. If black does not show, then it's more likely legal, except possibly having the black cover underneath. I would say that you should ask this in Q&A for an official answer--and if it's ruled legal, print and bring the Q&A with you.