TM Absol?

I don't like glassy pokemon, but it seems you are lacking in the area. At least with a fast TM you know you will get the desired move. If you don't mind glassy pokemon and want to power it up, I would say it's a good use of a TM.

Fast TMs are easy to come by. If you are gonna use that absol, then go ahead and use that FTM. Your tyranitars need charge TMs anyway so its ok.
Wouldnt recommend spending dust on Absol, unless its your favourite pokemon or you have already 6 high level dark Tyranitars.

Absol is a sheet of paper under the best conditions. I have a shiny and really want a reason to power it up. But there isn't one.

I don't recommend you spending any resources on it unless it's really just a favorite.