[This SE QA](https://english.stackexchange.com/questions/139316/what-is-correct-blazing-fast-speed-or-blazingly-fast-speed) explains that both *blazing* and *blazingly* are valid English words (despite what my spell-checker claims).

Can anyone explain why they are both valid, and the difference between the words.

In context, what is the difference between *blazing fast* and *blazingly fast*?

**Bonus Points:** The comments in the referenced SE question indicate regional differences in the use of these words. Does the meaning differ depending on where in the world people use these words?