I understand it was a joke. But right now it just seems to cross the bad taste line. I think the OP knew it would be taken racially, if it was just a poke at Justin Blackmon it wouldn't read "the Blackmon". Afterall you wouldn't say sack "the Brady". Ask yourself if Riley Cooper wore a T-shirt saying this the day before playing the JAGS would it be seen as just a joke.
With all tension from the Trayvon Martin case still in the air it just seems in bad taste. In fact with the Aaron Hernandez situation still raw using the word "kill" when referring to anything football might be in bad taste.
Can I say "kill whitey"?
Hey, shady baby, I'm hot like the prodigal son
Pick a petal eenie meenie miney moe
And, flower, you're the chosen one
Well, your left hand's free
And your right's in a grip
With another left hand
Watch his right hand slip
Towards his gun, oh, no