I don’t remember but yes, the minus sign would make sense, since I had 3/4 heart to start with so it would do the math. But I think it didn’t occur to me that symbols could be subtracted, unlike figures.
(usually I’d prefer a good old -2 HP, but you’d have to explain that 1 heart = 4 HP and that’s harder to follow)
Maybe the damage heart could be in a different color, as in Pokemon Ruby/Sapphire contests:
Or there could be a transition [heart before] -> [heart after] so there is no need for negative values.