I must question, WHY the necessity of getting the Black Card. Upon status-match trade up, you'll get a comp room, some free play, and some comp dollars. Black Card will allow access to the Borgata Amphora Lounge (for a fee, from comps) --- but, other than that, unless you plan on giving Borgata "proper play" --- Black Card alone is not going to get you anything in the way of future rooms etc. etc.
Getting a Harrah's diamond card playing $25 blackjack is virtually impossible. I could give you the figures, but it's basically not possible, due to the fact that earning 11,000 tier (requirement for Harrah's Diamond) would take forever to earn that many tier points. (Harrah's gives very few tier points to under $100 table players)
If you play tables, earning diamond at Harrah's would require significant table play at a $150 (or preferably above $150) average bet @ the tables.
Easiest (and most risk intense) way to earn Harrah's diamond is via slot play. Best way (though, earning is not as robust as slots) is via video poker (high denomination, if you are in a rush
) Atlantic City still has some decent video poker inventories, which makes this quite possible -- without risking a ton of money (or course, dependent one having a proper bankroll for the game played).