Blasters are limited by line of sight and they cannot fire at anything higher then themselves. They can sometimes destroy creeper not in their LOS if they fire on a square that is adjacent(or very near) to it and the square outside their LOS will be counted towards the squares they damage.

To shallow creeper a blaster damages 8 squares dealing 1ccc damage:

This will happen when the creeper is up to 1 elevation level deep. On the left you see a blaster firing into very thin creeper and damaging 8 cells. The damage of this shot is probably less than 1 ccc (8 x 0.1 ccc). On the right you see a blaster firing into creeper of exactly 1 elevation layer deep. The damage of this shot is the maximum you can get: 8 ccc (8 x 1 ccc).

For deep creeper a blaster will damage 7 squares dealing 2ccc damage to the square it hits.

This will happen when the creeper is more than 1 elevation level deep. On the left you see a blaster firing into a pool with creeper of about 1.8 layers deep. The shot does the maximum of 2 ccc damage to the cell it targets, plus a maximum of 1 ccc to 6 neighboring cells, so in this case the damage is 1.8 + 6 x 1 = 7.8 ccc. On the right you see a blaster firing into creeper of exactly 2 layers deep. Again the shot does the maximum damage of 2 ccc to the target cell, plus 1 ccc damage to 6 neighboring cells, which makes a total of 8 ccc (again max. damage for 1 shot). When the creeper would be deeper than 2 layers, let's say 3 elevation levels deep, a shot would do exactly the same thing, leaving some creeper in all damaged cells. The maximum damage of 2 ccc for the target cell is also the reason spores with an intensity higher than 2 can't be taken out by a blaster with 1 shot when hitting the surface.

If you drop a blaster into creeper damage will be done the following way:

On the left you see a blaster dropping into shallow creeper; 8 cells are damaged in this pattern:

111
110
111

On the right you see a blaster dropping into deeper creeper; 7 cells are damaged in this pattern:

111
020
111

One blaster shot costs 0.2 energy, so it can fire 5 times on 1 unit of energy, which makes the maximum damage per unit of energy 5 x 8 = 40 ccc. When you realize that a mortar shot does a maximum damage of 42.4 ccc per unit of energy, you can easily conclude that dropping blasters into a pool of creeper is just as effective as using mortars. Of course this isn't the case when you let them target shallow creeper at the edge of a plain. However a fully charged mortar can do up to 1152 damage as it has more ammo capacity whereas a blaster can only do 400. So when using them away from base mortars still win.

Blasters can benefit from "trough the wall mechanics"

This is originally information and screenshots made by UpperKEES. Link to the forum posts about this:
http://knucklecracker.com/forums/index.php?topic=5081.90