How do I get rid of fescue without harming my bluegrass?

If it's Tall fescue, paint the leaves with RoundUp using a sponge brush. Do this when the Tall fescue has grown taller than the KBG. Be careful not to get RoundUp on the KBG. It will take a couple of applications to kill Tall Fescue. There also is a selective herbicide that you can use to spot treat Tall fescue in a KBG lawn. It is a difficult herbicide to work with and there is some damage done to surrounding KBG, but it will recover. This herbicide is chlorsulfuron.

If it's a fine fescue, then they may decline in full sun when mowed frequently. The fine fescues do not like frequent mowing and you may advantage your KBG to take-over with this cultural change. Just keep the KBG well watered, proper fert and mow frequently. If the fine fescue is in shade, there is no know way to remove without injuring KBG.

Not what you want to hear, but is is highly unlikely. What you could do is to overseed with more bluegrass or bluegrass mixture to try to crowd out the fescue, but this would take several seasons and probably not likely or worth it. Your best bet is if it is really a problem is to till up everything spray the ground with a groundclear herbicide such as triox or roundup then overseed with your bluegrass (though i would recommend a bluegrass mixture instead of straight bluegrass as it is highly sensitive and fine)

It is not possible