Facial scar ... hmm. Not my area of expertise. They're usually quite small and superficial so I suppose you'd probably get some odd colouration, swelling and reddness rather than a bruise. More likely to open up again, TBH. Again, dependant on speed of blow and prior trauma. I'd be more worried about what was happening under the scar ... if it's around the eye then the orbit bone will be more layered, so may damage the eye or cause more facial swelling. if it is on the cheek then again you'd probably open up the scar again.
Good luck with it!