Nobody closes the curtains on tropical fish in the wild, so a bit of ambient light overnight is unlikely to cause a problem. Depends how much light though.
Swapping 45W of white light for 45W of blue light isn't going to do much good I imagine.
Just for reference, my moonlighting is so dim you have to sit there for about half an hour in total darkness before you can see anything worth seeing. But then it really is worth seeing. My Plec and BGKF come to life at night, the tank is a totally different world.
If you don't have some moonlighting, for the sake of £10 you're missing out badly on an unseen aspect to your hobby. Not sure the fish care, benefit, or suffer TBH.
My view is that switching any light on and off in the middle of the night is going to frighten your fish a lot more than leaving it on, but like I said, careful it's not too bright.
For me the test would be if you can sit there in a room lit by the main light, switch it off, and still make out anything other than the odd bit of movement in an otherwise dark tank- then it's too bright.