Why did you guys start talking about Garchomp, when the question was about Alakazam and Gyarados?
UU has decided to ban Alakazam and Gyarados, thus putting the two in BL. When they're banned, their megas are too regardless of whether or not they were ever usually in the tier. Banning a Pokemon with a mega evolution automatically means banning use of the said same Pokemon with its mega stone, and nothing else makes sense. If Mega Alakazam and Mega Gyarados were UU by the time Alakazam and Gyarados were banned (ignoring how weird it would be that the megas weren't then looked at for the sake of my point), then no one would have said the megas should have moved to OU instead of BL simply because they weren't the ones actually getting banned.
It makes sense with Mega Latios and Mega Garchomp to have their megas listed in OU because Latios and Garchomp are there. It makes no sense with Mega Alakazam and Mega Gyarados to be OU when their base forms are BL. This seems like a no brainer to me.
The reason it hasn't been done, I assume is just because whichever tool Antar is using to determine drops doesn't check BL tiers, it checks the tier below, in this case UU, which isn't an issue from a practical perspective anyways.
Even if you disagree with this, Mega Alakazam is currently listed OU and Mega Gyarados is listed BL, which tells you there is an oversight somewhere.