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"?
"There's a man goin' 'round takin' names And he decides who to free and who to blame Everybody won't be treated all the same"