As for unsolicited queries and how agents react to them, all you can do is keep trying. I don't mind to leave Belgium and live in L.A but the only problem is that I don't have a working permission. I'm afraid it's VERY tough. I would like a description of the follow-up postcard that actors send to casting directors and agents. The postcards you refer to are pretty much whatever you want them to be.
Most people put picture and words on one side, and mailing address, etc. on the other.
The cards are 3x5 or 4x6. And, sure, clever is fine, so long as it's not in bad taste. I am a 16yr old aspirimg actress, also African American. There is NO difference between minority and non-minority in terms of becoming an actor. And, yes, we look for the exact same thing.