its the water
I say it is the water. Not the bucket, not the lighting. The ocean is blue, clear lakes are blue.
When I aged water for my discus in a large white container, it was blue. In my white bucket it is blue.
Now, I do wonder if this is due to looking down on the water and not through the water, as in a clear glass, but it does require a lot of water for the blue to accumulate enough to notice it. 12 to 14 inches will show blue, ceartainly 3' shows a ot of blue, like the big barrel I had, but an inch or two will not show blue.