It does vary by tire brand, but " blooming" also occurs most in newer tires, and will get less and less as time goes on. Another factor that exacerbates it (aside from harsh cleaners that remove the outer layer and promote more to come out during flexing) is humidity/moisture. It's also referred to by people int he tire biz as "micro wax". It works in tandem with competitive absorbers (carbon black) to stave off ozone and UV, respectively.
As stated by rjstaaf, It is normal, and it's better to use a mild car wash shampoo on your tires than to strip the outer layer and create pronounced blooming from occurring.
Ideally, you want to keep those antiozonants in there as much as you can, and not deplete them prematurely. Cheers.
