Blue Marker is a 
Medium Marker in the game. He was added on November 6, 2021.
To obtain Blue Marker, go to the Forest and walk up the gray staircase to the left. Then, climb up onto the tree stump and use the vines connecting the trees to cross over to the next hill over.
Scale to the highest point of the other hill, and jump down the side facing the mountain onto a tree located on a lower section. Drop down to the base of the tree, where Blue Marker will be hiding.