You are in a bit of a spot as Seamaiden said, but there may be options. First of all, when you say blue spot, are you speaking of Cephalopholus argus? If so then this fish get's quite large. Around 16" or so. Expect it to reach about 8" before it's growth slows down at all in your tank. It's not altogether aggressive, and I've kept them in community situations without problems. Most of the time there are issues when you attempt to mix them with other hinds (Cephalopholus species) but even then, it can be done. I had a C. miniata and a C. argus together for a time. They chased each other for a day, then all was peaceful.
Some options include, but are not limited to, the yellow tang, rabbitfish, lionfish (yes, you can put lionfish in a reef, same caveat as the grouper) or a niger trigger. Ha!

Bluethroats, crosshatches, etc. Your tank is actually small for a full grown niger, especially with corals and such taking up swimming space.
Other options, frogfish, scorpionfish, etc. There is no rule saying you can't have a predatory reef. I thought about it myself when I was planning an in-wall 300 gallon for my latest creation. I decided upon a 150 gallon though, so scrapped that idea.
Really, what your main concern is going to be is waste load.
Cheers
Jim