on a centos system

in file mtab - does anyone know what vg_black-lv_home is ?

Its a dev/mapper name but I'd like to know where its from.

thanks

You should look at output of these commands:
pvdisplay
vgdisplay
lvdisplay

Ostensibly it is a Logical Volume (LV) named "lv_home" in Volume Group (VG) named vg_black. You didn't tell us the entire mtab line but I'm guessing this is mounted as /home or something similar. If you run "df -hP" it will show all your mounts including the devices from which they are mounted.

The commands dc.901 told you to run are LVM (Logical Volume Manager) commands that will show you what it has defined as Physical Volumes (PVs), VGs and LVs.

If you're not familiar with LVM there is a man page for it - just type "man lvm".