From an Instagram thread where I asked my friend the same quesiton...

Me: "Hey there, you're latest post is all black. Is that a reference to the civil unrest & injustice in the USA at the moment? I'm a bit out of the loop being out of country."

My friend: "It is - in short, people are “muting” themselves on social media to stand in solidarity with our black peers and to amplify black voices right now"

It's a well intentioned, although poorly thought out, way of "muting" oneself online to show solidarity with the BLM movement and to let black voices share their experiences and vital info. Instead it actually drowns them out, obfuscates useful information and turns your various timelines into endless black nothingness. Although I can see the value in hijacking social media for a moment to amplify a single message, some BLM leaders would rather it not occur for the aforementioned reasons[0].

[0] - https://twitter.com/BlairImani/status/1267706347753213955