In your constructor for your block
This thing:
add this to it: setBlockBounds(0.25F, 0.25F, 0.25F, 0.75F, 0.75F, 0.75F);
The numbers are (minX, minY, minZ, maxX, maxY, maxZ)
and then in the class add
public boolean isOpaqueCube()
{
return false;
}
Hope this helps