Chicago cops now tell TMZ they're not sure if the man arrested for allegedly having oral sex in a public park is a member of the Blue Man Group ... but a rep for the Group says the dude has never been part of the outfit.

Earlier today, police released the name and occupation of the suspect, Darren Stephens, who told them he was a member of the group. A spokesperson for the Blue Man Group says "The person arrested is not currently and never has been a member of the Blue Man Group."

We called the police again, and they say they have not conducted a background check on Stephens, and that "people lie on arrest reports all the time."