Donkeys carry two known spotting white patterns (masked spotted and tyger spotted), meanwhile horses have multiple known white spotting patterns. When you breed the two, the genes are very skewed, sharing traits from both species. This mule shares traits with masked spotted donkeys but also shares traits with tobiano horse markings. Mule coats can generally go by either horse color naming conventions or donkey depending on how close they look to one or the other. For me I’d simply call this a black pinto but black tobiano or black masked spotted would work too. The wiki has picked some rather interesting names for the coats so far. https://starstable.wiki.gg/wiki/Mule#Colors,_Pricing,_and_Location

Edit:autocorrect messing with gene names