You can set calibre to use
Bold text in mobi output during conversions by inserting CSS in the Extra CSS field on the conversion Look and Feel window. Depending on the existing CSS for the book, you can add
Code:
body { font-weight: bold }
and the text for the whole book will be bolded. However, mobi cannot embed fonts so I'm not sure how you managed to get a font other than the Kindle fonts to display. I suspect you merely saw a bold version of a Kindle font. Unless your Kindle is hacked, in which case I have no idea what it might be able to display.
BTW, I use the
body { font-weight: bold } CSS on all of my books (mobi and ePub) that I read with eInk screens. Otherwise, even the Pearl eInk screens do not produce text that is dark enough for my eyes to comfortably read for hours.