Blue stone

Blue Stone, or Blue Rock is a type of pagan sacred stones, widespread in Russia in areas historically inhabited by both Eastern Slavic, and Volga Finnic tribes. Unlike Sledovik, Blue Stones did not have major hallows on them, and were venerated in a simpler way: by pouring water on them, or leaving food offerings. Some of the Blue Stones are still known, and to some extent venerated by local populations.

If used as a personal name, Sin-Kamen usually refers to the most famous sacred stone of this kind, located on a shore of Lake Pleshcheyevo near Pereslavl-Zalesskiy. While in the majority of cases, the stones belonging to the Blue Stones type are black or dark gray, this particular stone does indeed look dark blue when wet.

Source: Wikipedia