Yes. IIRC, the products have to be business OR personal - unless your business cards are specifically tied to you as the primary. That is, if you have several personal Amex products and your spending is above $250k PAID (not balance on credit that isn't paid), you qualify. I believe they do count spending on business IF the business cards are tied to YOUR credit to begin with (ie SS#, EIN #, etc).

Yes, of course additional cardmembers spending qualifies, as all that spending is actually on your account, not there's. Hence all the business black. So theoretically, you can pool spending to get to black - although if you're doing that, your income might not be where it should to qualify in the first place.

Its does vary, but Amex relies on their internal sliding scale to determine if your income, credit report and spending qualify you for black. From what I remember about my initial conversation with an incredibly forthcoming CSR a few years ago, low to mid 6 figure income, which is verified by Amex.

BTW, there are a few experts (as least it certainly seems they are) on the subject on FT that probably have a lot more to contribute here than I. MIA, Kagehitokiri, and Centurion all seem to know alot about black as well.