Weevil

Best answer as chosen by Asker / Ratings
Black soul gems can trap the souls of NPCs, whereas normal soul gems cannot.

DivineAngel

You can also create black soul gems as well if you are interested. To do so you need an empty grand soul gem. These aren`t easy to find but they be bought from certain alchemists though they don`t come cheap. Once you have one, travel to a place where there is an alter such as Dark Fissure, Fort Istarius, or the Elven ruin of Wendelbek (I think there is another one but I cannot remember what it is called). If there isn`t already a purple light shining down on the altar wait until there is (it is the shade of the revenant and happens only once a week. You may be familiar with this if you happen to be in the mages guild and are working for Raminas Polas at the Arcane University.) When the light appears, place the empty grand sould gem into the altar and cast the soul trap spell on it. Ta-da! You have a black soul gem.

As for using the collosal soul gem on Count Hassildor I think that must be a glitch because you cannot use it against him. If you place the cursor on him it changes to a crown which donates the fact that he is one of the main characters (as are all counts and countesses) so cannot be killed.