[mrsquash]

I ran into a file like that several months ago. I used calibre to convert the mobi to epub, looked at the stylesheet.css file, and found that one (or more) of the styles had a line that ended in ":blue". I think there was something about "text" earlier in the line but it's been a while. I deleted all such lines from the css and converted back to mobi and the problem was fixed; all the "blue" text, which had been grey on the kindle, was then dark.

edit: It occurred to me that this would be a lot easier now with calibre's "inspect" feature for epubs. If you're not familiar with it, you just open the epub in calibre's viewer, put your cursor on one of the grey areas, right-click and choose inspect. A window on the right will show you the applicable style for that section and what its parameters are. Should eliminate the guess-work like I had to do back in the old days.