Blue will mess up your night vision more so than red... but shouldnt be a problem in a fish tank when your seing reflected light. I beleive what people here mean by 'mess up your night vision' is when its dark at night your eyes adjust to the darkness so you can see in the dark better. If you turned on a 100W light in your room or staired into a flashlight for a moment, then turned it back off, you wouldnt be able to see in the dark for a while until your eyes slowly adjust back to night vision. There is nothing permanent, its just that it takes your eyes a while to adjust to see in darkness.
I use some ultra-bright blue led's (as bright as the ones found in the expensive flashlights) and if I opened the hood and stared at them for just a second or to it is very bright and would mess up your night vision. However, just viewing your tank illumiated by bright led's is probably not going to do anything to your night vision. And if your going to purchase moon lights then most of the commercially available ones are not that bright anyhow.
Blue is also a color that reflects very well if you have blue or irredescent fish so it usually looks the best. I dont think moonlights really serve a purpose other than looks.I havent seen any studies that prove they reduce stress or anything like that. I have not tried yet but I am thinking about putting some red leds on one side and maybe blue throughout the rest, just to vary things a bit. Heck, could probably throw some other colors in there too, just make sure your fish dont seem to hiding from it, as long as thier out and about then it should be ok.
