Ok, I'll bite on this one. I noticed it is laying dormant!

That hole is pretty small to worry about but I guess you want to keep dust out.

My first thought was a dab of black silicone seal but even clear would pass.

The next thought was a pinch of HVAC duct seal that is grayish to black. Some Ace hardware store carry it in their electrical departments.
Also in their speciality drawers of the nut & bolts department you might find some tapered rubber plugs but that is an awful darn small hole! A pointy, quarter inch one might be plausible.

If you want to go totally black I got two options. One is piece of black electrical tape. Yes, the same stuff Tom and Ray of Click & Clack fame use to cover CEL's.

The next one is for the adventurous, in a since, you get to transplant another piece of your car up there to look at!

On the drivers side there is your threshold plastic strip that holds the carpet and door trim in place.

You pull that up and peel back the carpet away from beneath the fuse panel area. Somewhere along there,holding the wiring harness to the floor board, is some black sticky gump that you can pinch off and plug that hole with.

If you go this route, I'll know that you will reach to the depths of your cars soul to find a solution, a true to his colors, Volvo mechanic! (:)

Phil