Hi @alexdory, thanks for links and travel tips :) In fact, Romania ranks high on my list for the next travel destinations.
Your explanation of the blue color of deep water in general is correct! In this case though, what adds to the effect is the scattering of the sunlight at tiny carbon particles that are dispensed in the Blautopf's water. And high energetic wavelengths scatter the strongest, thus an additional boost of blue.
Thanks for participating!