BiTgte_Blend and BRepFilletAPI_MakeFillet both create a fillet.
BiTgte_Blend requires that two surfaces intersect / penetrate each other. They should not have a common boundary edge.
BRepFilletAPI_MakeFillet, on the other hand, requires that they have a common boundary edge.
Draw-Script for tests with BiTgte_Blend:
restore face1.brep f1
restore face2.brep f2
shape qq C
add f1 qq
add f2 qq
rollingball r qq 15 @ f1 f2