I don't know what world you race in but it must not be Earth.
We have guys out here that are an absolute RIP in FF's. We have other guys who can just barely get them around corners. So, yes we blue flag same class cars all the time including GT1, FC, FM, etc..
Like has been said it isn't a 'let them by' message. It's a warning to be aware of a car close behind that looks like he's going to make a move on you.