I've had the same problem.
You can increase the resolution of the fluid contianer but it wall also affect the dynamics of the simulation. In other words, the explosion will probably look different if you increase resolution.
I think that if you decrease the amount of fluid/second this will also help.
To be completely honest, I never did come up with a great solution to the blockiness.
There are some explosions and a meteor animation on my website. Check it out!