Fate was a cruel mistress, if Casey took stock in that kind of anthropomorphication.

Because his snacks were screened. It wasn't uncommon for Slytherins to have such flourishes in their rooms, like bedside furniture brought from home or secret stashes of lavish treats. Casey's were not elaborate in this regard, being mostly dried food mixes that kept, bare bones grains and things he could actually stomach.

Yet with an alien but horrifyingly recognizable taste....

There were few fruits, dried or otherwise, that did not have some bizarre magical allergic reaction associated with them after consumption. In the broadest terms of categories there was a thematic element to these reactions of Casey's, the absolute worse of which:

Blueberries.

Which left every inch of Casey's skin, but not hair follicles, a deep azure like he had been injected with paint. This tended to extend beyond the duration that the blueberries were in his system. He would be exiting his fourth year splotched with blue juices at this rate. As the reaction had just happened Casey was not looking to have anyone notice. To his embarrassment, the only temporary solution would have to be wearing makeup, and the embarrassment of that was not hampered in the slightest by his only having a certain amount mixed. More could be made with rather common ingredients to be checked out of the Potion stores.

So as Casey slunk around the halls during the brief weekend break in the schedule of exams he really wished he could use his invisibility cloak. There was nothing stopping him but his own critical mindset. Though he'd stealthily been through the castle halls before under it, it would be far worse to be discovered that he was using an invisibility cloak in the middle of finals and thus that happenstance was a distinct possibility should the coal be used.

Had he been paying attention to his reflection in suits of armor he passed, Casey would realize that some test smears of the makeup that might make him appear normal for the duration of this allergy covered his nose and cheeks. A weird skin tone highlight to his blue predicament that arguably made his face look worse. Casey was only focused on checking stock at the old infirmary first aid outpost, to see if there were other components there.