@Miss Texas 1967 you are correct when you say that they only did a blood test since DNA testing was not yet available when the paternity suit episode was being broadcast. It was still a few years off until DNA became a thing.
However it is possible that Ray was never a candidate for John Ross paternity. I mean it would depend on when the baby was conceived and they indicated that it was later on the show.
It is hard to determine how much time that passes by in an episode when it's centered around one plot but I would assume that some weeks might have passed between the beginning and end of the Black Market Baby episode considering that Sue Ellen tried to adopt a baby that this girl was pregnant with. I think it would take JR some time to get wind of all that.