I think it looks cool, why get rid of it?
For about a minute.
I had this problem with steam pouring out which looked really cool. It is not DG related as this happened to me in December. Just unequip your armor.
1. You'e right, it's not Dawnguard-related.
2. It's also not Blackreach related. It's probably a game engine bug.
3. It's not isolated to just the PC. I saw a bird flying around with the effect too.
4. Unequipping armor doesn't work. I tried the command unequipall as well, but I got a CTD.
It doesn't look like anything but stop magic effect commands work. That would take an hour. Instead I just reverted to an earlier save.
If anybody ever finds a solution for this bug, I would like to know, but I think in the future I'll just be wary and have a lot of saves on hand.