The mouse is a suboptimal pointing device in several ways, but it's one we use for various reasons despite this. We devised a keyboard that takes both hands and very nearly all ten fingers to use properly (you can type with only one thumb) - yet we then added a gizmo that forces the user to take one hand away from the keyboard to use it at all. Oops.

In Dilbert there was a monkey who used the mouse with his tail; that would be practical for aliens with appropriate appendages. Humans could use a foot pedal/joystick/something system, but we don't. A few head-mounted systems have been tried, but have never caught on.

For some purposes the blob would be quite practical. Maybe you hold it in one 'hand' (it could be sticky to help with this); an accelerometer array like in the Wii would let the user easily do fancy pointing and selecting, while a chording system would allow the equivalent of typing. This might be frustrating if the aliens' hands did not map well to human hands.