Rhodes hit his seventh goal of the season when he slotted home with the game into injury time.
Before that, Rovers had struggled to get the break-through as Watford set about getting a point.
Black told Sky Sports: "If you'd come to the game and watched the first 80-odd minutes you'd have said, 'Oh, he's not done this and he's not done that'.
"But to be fair to the kid we don't, at this moment in time, give him the opportunities that his finishing deserves.
"However he's a top, top finisher and that was typical of a top finisher. Not in the game for spells and it was a wonderful finish."