Can't speak for what Mass EPO's will do, but I can tell you what Maine game wardens have told when checking birds I thought were hybrids.
(1) If there is even a hint of green on the head, they won't call it a black duck. May be an immature mallard, may be a hybrid, but it's not a black.
(2) If there is even a hint of 2 white lines on the wing, it's not a black duck. Again, might be hybrid or might be mallard, but not a black.
(3) If you can clearly distinguish (and the warden agrees with you) hybrids from mallards, they don't count as part of your mallard limit, but as "other" ducks.
(4) They encouraged harvest of hybrids--"get those buggers out of the gene pool" is a direct quote.
Now, that's just one warden from another state, so take with several grains of salt. I remain conservative, and have been checked since then with ducks I thought MIGHT be blacks, only to have the warden tell me it was a hybrid.