Black Slimes

I tried using google about these little guys but, a witch just flew over my slime hut and this happened...

Anyone know what is up with these little coal colored slimes?

 

Also, one occasionally flashes between blue and black... Very confusing thus-far

 

Witch?, is that the big black "crow/bird" I sometimes see passing overhead?

 

From what I read from a previous case, the black slimes have a higher chance of dropping coal. (though I haven't experienced it personally, so I can't say for sure) You could try breeding them and killing them for a steady supply of coal.

 

Drops were Slime, sap, coal, Bixite, and Neptunite

 

It happened to me to a few days ago too, along with the above I tend to get diamonds often from them too

http://images.akamai.steamusercontent.com/ugc/548682681773831955/F12EC3A6820FFA0DCF38140D6317C9125A77ABBB/