Its not that black is bad, its just no where near as good at producing combat triggers as blue, which is what this card is about. Blue is amazing at both protecting individual threats via soft counters, or sneaking threats in for combat triggers via evasion granting effects (flying/unblockable granters) or with bounce.
Sure, black can just try to spot remove everything larger than it, but it can't protect grim flayer, it can't make it unblockable or give it flying against a developed board, and it doesn't have aura's to directly buff its trample evasion. This is awkward for a card that somewhat struggles to live up to its full potential if it can't connect once or twice, and seems to damper some of the fun.
Like I said, I really like the card; it just has a few issues that should be considered, and I would have much preferred to see this style of card in Simic, where it would be exciting due to how well it fits into that guild's tempo plan.