There are two different Blasto corals - Shamrock, it sounds like you and I may have the same coral. I typically comes with a single head, and just kinda gets bigger. Making sure that your calcium levels are good, will ensure a nice growth for this.
The other Blasto, like Carey, and I believe Joy has, are multiple head blasto's that grow in colonies. These are like polyps and zoo's that will continue to spread, get more heads, and expand.
Saltwater Aquarium Corals for Marine Reef Aquariums: Australian Big Polyp Blastomussa - Aquacultured
These links show the difference.