"Digital Blackface" is such a terrible way to word this, given the seriousness of what blackface actually is (as a European with the history of it here). That sits at a borderline over the top way to describe this.
As for GIFs, I'd like to think 95% of people when they want to express something via "video" (as that what GIFs really are) just search for the emotion they want and post what they find the funniest/most appropriate. I doubt most people search for "white guy laughing" or "black guy laughing" instead of "guy laughing" (unless you're going for a specific actor/image you can't remember the actor's or show/movies name). When searching for a GIF you tend to get the most viral or popular stuff first, and that can explain why it might tip to an over usage of a black person, or a white person, or whatever person it is. I mean, think of the most popular ones, someone like Michael Jordan is going to be up there, so he's seen more often than others. People tend to favour being quick to express themselves, and not sitting through pages and pages of GIFs (5~10 mins) just to find something specific.
The rise of GIF posting is probably more to do with Twitter/FB and even iOS/Android keyboards including GIFs for people to express themselves. I'm not sure if asking people to veer towards only using emojis/gifs that represent their own skin colour will be all that productive a lesson to try and teach. A lot of human emotion/expression is supposed to be spontaneous, so people don't think long and hard about the skin colour of the person in the GIF they are posting, but more-so how suitable the reaction (or acting) is for whatever emotion they are wanting to convey. Such as laughter, sadness, anger, etc.
Therefore in the majority of situations it's simply human beings using human expression. An evolved form of "LOL", given the world isn't on 56k dial-up anymore and videos (GIFs) can be loaded quickly. Of course, overuse of GIFs can be annoying, but, it's the internet, the majority of it is annoying! And yes, as others in here have said, something like Twitch chat does end up becoming a racist play-out of using memes and GIFs on purpose to express abuse. That's not the same as someone simply using The Wire actors in GIF format for a joke/good use of an expression.