Hook: Wall Small (Green Square)The Blaster Mark decoration fits into a small green square wall hook.
Hook: Wall Medium (Blue Square)The Blaster Mark decoration fits into a blue medium square wall hook.
How to Get a Blaster Mark Decoration
Cartel Pack Decorations
The Blaster Mark decoration can be dropped from Underworld Alliance Packs, which were released on the Cartel Market and bought with Cartel Coins. Cartel Packs, including the Underworld Alliance Pack, include a random selection of items from the Underworld Alliance collection of items. You aren't guaranteed to get a Blaster Mark decoration from a Underworld Alliance Pack, but it is one of the few ways to get a Blaster Mark deco apart from buying it on the GTN from another player.
If you're looking to get a Blaster Mark, the easiest way to get it is to head to the Galactic Trade Network (GTN) in SWTOR and buy it with credits from another player. If it's very rare and not available on the Cartel Market, you could try buying from the GTN and hope you get it.