maybe you're not that much into the realm discussions, cause there are/were tons of QQ of Blackrock players in terms of "low arena activity" and low popultion, so this "crossrealm" decision/idea is based on that mostly like. Same goes for Lordaeron.
http://armory.warmane.com/ladder/3v3/Blackrock is just an example of what im talking about , a "pure pvp" designed realm have less 3s ques then Icecrown, less population (that means less interest of join that realm nowdays)
Same goes for 2.
And Blackrock even have a way better MMR compared to Icecrown that allows to get higher rates fasters...
A really good move would to make MAYBE only 3s or soloq Crossrealm, in order to not merge all these metagames and do mistakes during it (cause u know, you change X , Y will be the problem, you change Y , Z will become the problem)
Blackrock will never be a donation realm, or at least not even close to Icecrown, probably whoever will donate for Shadowmourne on Blackrock will just be the top tryhards on the servers, altho i rather understand better allow donation on blackrock for smourne (higher price then Icecrown, for obvious reasons) then see all gear on Icecrown being nerfed just to be as closer as possible to Blackrock wannabe standards.
OR, it will be like a well known realm, that on the I80 the Shadowmourne is like 20euro, and everyone will just join Blackrock to donate and do arena with it...