Blackening isn't just a spice rub. It also refers to the technique. Done in a screaming hot CI pan. I mean so hot the pan bottom looks white. Using a blackening spice rub on a chicken roasted in the oven, doesn't justify calling it "Blackened".
You might be able to use the blackening technique on a whole chicken if you "spatchcock" it and use a mallet or debone it to get an even cooking level. You would probably still have to finish it in the oven though.