I have had several blennies over the years, the starry blenny was by far the most aggressive. Especially with anything that was even similar in size, shape or color to itself. I would not do it either.
I have tried this also with horrible success even knowing they were the opposite sex. Thet almost killed eachother and i had to rip apart my tank to get them to stop fighting. I felt terrible for doing it since it is my favorite fish of all time. More than half its tail was missing :'(. He lived for another 3 years and i returned the female to the LFS.