I've found Blackhawk's clip-on pouch a little bulky for concealed carry, personally. If the mag is in the pocket it i never going to be as fast to get at as on the hip or in an inside the waistband pouch, though holstered pistol AND magazine IWB gets a bit uncomfortable for some folks.
Glock's own inexpensive plastic mag pouch conceals reasonably well, and leaves your pocket free for other things...like a pocket holster for a backup gun.
best,
Mas