Bluish is a polysemantic term that is used in various fields of science and technology.
Bluish in the photo: Unfortunately, I don't have a photo of the blue spot, but I can describe its nature. The blue spot is a small depression in the hill. This place is shaped like a funnel with smooth walls. There is little water here, and it usually collects along the edges or at the bottom. Because of this, the bottom is not visible into the water very well. The blue spot was formed due to soil erosion. As a rule, the shore of such a place is usually surrounded by stones, and only close up are traces of erosion visible. To see this place you need to climb a steep slope. Then you will have a view of the area for several tens of meters around. You can taste the water if you have no desire to engage in fishing or hunting. Another blue place is located far from populated areas. Precisely because people rarely appear here.