Have you ever had just one color of paint refuse to dry?

I'm convinced that something is wrong with my cerulean blue oil paint… I used a pretty concentrated cerulean in the sky part of a landscape, and now that one part of the painting won't dry.

It's the same brand, same tube that I always use. It usually dries to the touch within a few days. The other parts of the painting have dried. Only the blue sky is wet. It's not the most recently painted section… in fact, I painted it first!

Here's a video (it's boring, sorry, and I have no idea how to edit videos… is that a great pitch to watch it or what?):

I was hesitant to share this conundrum on my blog, but who am I kidding... painting is an ever-evolving skill and artists are always honing their craft. I'm grateful for all the lessons I've learned so far, but I know it's nowhere near the level of knowledge I'll have in 5, 15, or 50 years! I like to keep this blog pretty casual and very honest… so… there it is, for all the world to see: a hiccup. I'd like to learn from it. But what caused it?

Can y'all think of any reason why this would happen? I am baffled. Help!