Oldish Sketch. .o. How my Ponysona got the black stains on her nose and rump.

Basically, she was on an excursion to a factory with her school group. During lunch, she wandered over to look at the chemical vats, leaning heavily on the containers to look inside.
The resident class bully pushed her, leading Cross Hatch to have her nose shoved into the black liquid. In her panic, she managed to tip the whole vat over, before falling on her rump in the black puddle.

AND THAT IS HOW SHE GOT BLACK STAINS ON HER NOSE AND RUMP. The bottom of her hooves were stained, but they have long since been worn down of the colour.