They are not the same.

A musketoon is a short barreled musket. As such, it fulfilled the role of a musket as a military weapon but was shorter to contemplate some reason that the solider issued it needed a shorter weapon, that typically being that he was a mounted solider. FWIW, good examples of musketoons used by the US can be found in the older text United States Muskets, Rifles and Carbines by Gluckman which makes their nature as short muskets fairly apparent.

Muskets took a variety of projectiles, including balls of the same caliber as the musket and smaller balls. This might not make much sense, but for example a typical load might be one single ball the caliber of the weapon. Or, in other circumstances a load of smaller balls, or both (buck and ball).

Blunderbusses, on the other hand, were really a type of early "riot gun". I.e., they were a type of proto combat shotgun.