Probably reflection from the sky, cover the window to see if it is still blue or look at it at night.

It’s still bluish at night when the blinds are drawn. The window in the other bathroom is about 10 feet away from the tub.

I think it is a reflection as well, not a doubt in my mind. Do you have well water or city water? Have you had the water tested?

Reflection.

@millworkman City water. Good point about testing the water!

Is it blue also in a drinking glass ?

That's the natural colour of water. It's harder to see in a drinking glass because there is less of it, but it is particularly noticeable in a white tub. Nothing to worry about.

That looks like copper to me. If anyone in your family has blonde hair, they need to avoid green or blue shampoo, or they might end up with tinted hair. You may have staining in the tub, which can be fixed by cleaning more often or wiping dry after use.

I'd put an undersink filter in your kitchen and drink/cook with filtered water. It's pretty cheap. Make sure you get a good filter set.

Water is naturally blue and noticeably so against a white background.

As said above, water really has a natural slight blue color. The stark white(?) color of the tub accentuates it. My whirlpool soaking tubs in both bathrooms, which are a light ecru color, exhibit a very faint blue tint when filled (well water, not city supply).

Thank you everyone! You’re right the bluish color is normal. I tested the water using per millworkman’s recommendation. No issues or contaminants other than I have very hard water.

Our water looks bluish in the tub. Just take the bath for Pete's sake!

Bathe and pretend you're in the Caribbean, on a white sand beach with the beautiful blue turquoise water lapping at your feet. Ahhhhh.