As I said it is NOT a real problem in terms of a bug or a corrupted file. The converters are fully functioning. Most certainly it was the author of the ebook who made the text grey/black for better reading. The converters take the file and just translate the authors text markup into the other files dude.
html stands for hyper text markup language. ePub uses html directly and mobi is derived from it.
Try to open your ePub version with Sigil and have a look
