Storebought pasteurized/homogenized milk and Ultra-Pasteurized cream.
Storebought cultured buttermilk, left out in a covered, sterlized jar to thicken further.
~1 Week old store bought stilton cheese, blended with boiled/cooled water.
None of those are the culprit, I feel 99% confident in saying.
Recipe says little about temperature. Video says warm/moist environment, but gives no time. Various accounts I've read here have said 3-5 days at room temp (around 70 degrees). I had it a little higher temp (~75), no particular attention paid to moisture (cheap hygrometer says 60% RH, now that I test).
Somewhere herein lies the issue, I am confident. As I look closer, it looks like the only spots that are affected were on the top and bottom, where it was sitting on the plastic craft mats. I feel imagine that if I'd been aging on bamboo, it would've wicked away moisture, and the rest would have dried down to more of a rind already, keeping this at bay.
Worlock, I'd say it's likely that the wetness was a factor. (and don't worry about accusations -- It's all experiments, and the more feedback, the better. I've got a pretty thick skin. Unlike this &%#@ing cheese.)
I know I should just toss this, but I've put so much thought and effort in already...
I cut off all the weird looking spots, and will see if badness keeps popping up. If it looks clean, I'll let it age a little longer and see what happens (what's the harm? Aside from allowing blue-cheese loving flies to infest my home...). If it doesn't, I'll bite the bullet and toss it out.