Skyrim IV: Oblivion

[LordShotGun]

So, hey! Ijust found a black soul gem. How rare are these?

Much more common then in oblivion that's for sure! In oblivion you had to go to a certain spot at a certain time of day on a certain date when the planets aline with a SINGLE grand soul gem to transmute into a SINGLE black soul gem.

Also, I now know a bunch of words, but only 1 for each except for Fus Ro Dah. How do I increase my phrases?

Without giving out too many spoilers, you can only find a few of the word walls in some of the semi-main (companions, brotherhood, and such) quest lines.