Blizzard has a reputation among PC gamers for being roughly platform-agnostic by releasing their games on both Windows and Mac, but it looks like that may change with Overwatch.
Blizzard published a FAQ for the closed beta of Overwatch and it says the game is being developed for Windows PCs only:
Q. Will Mac users be able to participate in the beta?
A. We’re developing Overwatch for Windows-based PCs only, so a Mac-specific beta will not be provided.
It doesn’t say a Mac version will never happen, just that there won’t be a Mac beta. As the game is being developed for Windows PCs only, it means that any potential Mac version may be a later release. Datamined strings also show that PS4 and Xbox One versions are likely on the way as well, so it would be strange to leave out Mac support.
Aside from their older console titles, every Blizzard game since the first Warcraft was also released on Mac. While the earlier ones saw a Mac release later, for the last few games its been a simultaneous release even including Heroes of the Storm. Maybe someone will ask Blizzard about this at next week’s Blizzcon convention to get to the bottom of it.