User blog comment:BannedLagiacrus/Monster Appreciation Week: Black Diablos (5th Gen)/@comment-4732465-20180714140252/@comment-4911512-20180715014854

"The black color is used as a warning to other creatures, telling them to give her space, while Black Diablos is gravid or has recently laid eggs." It says this right at the top of the "Interesting Facts" section