That depends on what you may or may not accept as "Russian blood". Are we talking about nationality or ethnicity? I guess many Russians, born in Russia, children and grandchildren of Russians, have a lot of Viking or Tartar blood, and I don't think that make them "less" Russian. Considering how peoples have moved from here and there, who can say "I am 100% Russian" (or Nigerian, or Peruvian, or Chinese... not to mention Americans).
The question of Paul's legitimacy is a hard one... actually it was Catherine who in some writings stated he was not Peter III's son. Of course, that may be true... but I still wonder about some details, such as the following:
1) Catherine never showed love nor affection for Paul. If he was the son of a man who provided her love and confort (and sex!) when she was still married to Peter III, it would have been expected quite the contrary: to love and cherish a boy who could remind her of one of the few persons who treated her as a normal, young loving woman. However, she behaved toward Paul the way she would have done to a child by Peter, a man she despised and who behaved like a rascal to her.
2) Catherine, a German princess with no connection to the Romanovs, was not a legitimate ruler - denying Paul's legitimacy would have been a way to say "Oh, ok, I am not the legitimate ruler, but Paul has no better claim, since he is not the son of the Tsar"
3) Catherine was not Miss Universe, but was an attractive young woman, and Saltykov, the alleged father, was a good looking man. Poor Paul was anything but handsome... where did he got his looks from? Peter III ...? Not to talk about their mental balance...
Of course, only DNA could have cleared the point...