In view of LeBlanc said to Swain “You knew it before you sold yourself” in Judgement. So is it means LeBlanc knew Swain before Swain met Beatrice? or any reason?

I think she was referring to Darkwill, that he sold himself to Darkwill.
But yeah, they seem to have quite a history.

There is a small time, between Swain arriving at the hospital, leg broken, and his arrival in the military. My guess is, that in that time, he was serving the Black Rose. He saw it necessary to infiltrate the "enemy" from within, and thus, as a Teenager/young adult, he joined the Noxian Army.