I've seen a lot of vivid blue lakes, some in Romania. My friend @gabrielatravels had a nice article of it here and another of my friends wrote about it here.
From what I can tell, the colour is given by pure water, filtered by layers of Carbonate sands, which usually also make the lake bed. Because of the clarity you can see to great depth which means that only the most powerful wavelengths are reflected back to the surface (the blues).
It is also why we only see blueish stars in the night sky.
I can recommend (obviously) to visit Romania :D but also go to lake Bled in Slovenia, Montenegro has very nice lakes and Croatia has Krk and Plitvice. As for Switzerland, near St.Gallen there are a few wonderful lakes.
I have added Blautopf to my map for june-july.
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!