Related: http://typophile.com/node/83769

If a black is intended for display, as it is in most cases, shorter descenders would come in handy for setting lines tighter.

I'd certainly recommend rebalancing the ascenders and descenders as you don't want them to look different lengths. Normally in text fonts ascenders are slightly longer than descenders. At the end of the day, type families need to look as though they have the same proportions (which isn't purely a matter of linear dimensions but also of relative areas and overall colour). This means that my answer would be to compare the regular weight with the black weight, using example words with descenders and ascenders, and shorten/lengthen the extenders until they click across weights.

In my font Mint, that I'm currently working on, the Fat ascenders are slightly taller than the Medium, but the descenders are the same length.

@Frode: It could well be used for display, but I want to keep the 'Sensato' feeling, which requires a little long descenders/ascenders

@Ben: Thanks! Working on it :-)

ohh and, nina: I kinda forgot about that previous thread I started and your thread is intersting, but none of this says anything on the relation between descender and x-height/descender and ascender. Thanks anyway ;-D