How to design Bold weights
Does bold method differ between Latin an Arabic Script?
For Latin, to my experience, the best is to design a Black rather than a bold and use blend function in FontLab for example. This way you can adjust any time your correct weight for the bold depending of the situation.
Further to this practical detail, Bold is difficult to design because generally the ratio between white and black is 50/50: not enough contrast to be sure what to do with counters (width counters ratio between n, b, o and narrow glyphs like a, e, s).
Sanserif approach is not the same of the Serif approach. On the second case you will increase probably much more the heavy parts and leave the thinner parts very close to the Regular. For Sans, booths will increase in way you maintain the optical illusion between verticals and horizontals.
On a side note, I recall to have designed once, on the flight to Beirut (few years back) few glyphs for a remplacement of Anisette Petite Black of the introduction for speech. Despite that is not an appropriate design for a good Arabic type, Its still a good demonstration that weight should increase horizontally, never vertically. :)
(In answer to a question on ATypI mailing list.)