External Monitor Periodically Flashes Blue with 2016 MBP on High Sierra
I have had this issue when restarting my MacBook Pro while being connected to a 4K TV. I found that in addition to the blue flash, it sometimes decides to turn black for a fraction of a second during normal operation (with an external monitor attached).

The way I was able to fix this is by connecting the Mac to power from the wall. Having researched this extensively a couple of months ago, I found out that the video initialization (and apparently frame rate conversion) to the 4K monitor or TV causes the blue flashing as it undergoes the process of colour matching, scaling etc. I am not too sure why it is blue, but that's a topic for later.

I'll spare you the links and instead have summarized it here. This issue annoyed me at the beginning but since having my mac plugged into the wall outlet, I have not come across it again. I hope my interpretation of your issue was correct, please let me know whether having the Mac plugged in makes a difference in your case, I'd be interested in following up with you on this.

Addition
If you have access to another High Speed HDMI cable, perhaps you can borrow one, I suggest you try replacing it with the current. My guess would be that perhaps the adaptor is somehow degrading the signal. I personally have had issues with non-Apple adaptors in the past - especially when 4K quality or high frame rate is key. I'm certainly no expert on accessories but maybe it is really worth testing another adaptor.