Bummer.
This is problematic to me for a few reasons - the first and foremost being that he would be excellent in mono-Green. The Wolves combo with Master of the Wildhunt and his 3 damage is super relevant.
Not so much in BG, where he is just another card that kills Creatures. Black has plenty of cards that are functionally more exciting than Garruk. I am not sure he is all that great in my Ghave deck even, as the interaction with Doubling Season makes it harder to flip him, though I suppose he could just Lightning Bolt Ghave and take 5 damamge himself. Likewise, Glissa has little to no use for him.
I think the fact that he will kill the other two Garruks, who I think are much more solid and reliable, make Relentless the odd man out.
The justification for him being BG - well, the color identity rules specifically mention the color of the card, even though it has never been relevant since there are no colored cards without a mana symbol on there somewhere.
