The hints have been there for awhile. The Garruk-Liliana storyline. Webcomics. Garruk Relentless.
To me, this kind of cements it: We're going to see Garruk finally move into black in the color pie. In case you didn't know, the storyline is that Garruk has been cursed with the Veil (of something) by Liliana. This is how we got a partially-black Garruk Relentless, and it looks like the transformation is now complete. I don't know what this means for the future of the character or even what a new Garruk would look like, but I do imagine it will be fun to find out.