Moody Blues (or at least, Stubborn Cerulean)

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!