uhm, which type of Blasto are we talking about? Welsso's are pretty easy to frag using a diamond band saw, without I'd really rather not try it! If you have a LFS in the area, see if they have a band saw that you could rent for $5 or something. Depending on the growth, you can try the chissel approach between the columns, but you run a risk of cracking polyps in half which will usually cause the polyp to die.
Merletti's I've never fragged, so I'd rather not try and offer advice on them, but I would guess it's along the same lines-really, try and find a band saw, just so much easier. And yea, using a wood bladed band saw will be cheaper, and you can find really affordable ones, but getting a diamond one is well worth it-maybe see if you can get a couple reefers to chip in on it so you and some local buddies all have one to use when you need.