Blasts do not roll to hit.
When firing a Blast weapon, models do not roll To Hit.
If the shot scatters so that the hole in the centre of the marker is beyond the table’s edge, the shot is a complete miss and is discarded.
Is the only way described to actually "miss".
Blasts generate hits on units that have models under the marker, but they do not actually hit something as per the rulebook definition of
4. Roll To Hit. Roll a D6 for each shot fired. A model’s Ballistic Skill determines what it must roll in order to hit the target.
and they do not have to follow..
Note that the minimum roll needed To Hit is always at least 2. When rolling To Hit, there is no such thing as an automatic hit and a roll of a 1 always misses.
However blast is a special rule, and therefore trumps the core rules of shooting.
A unit takes a hit for each model that is fully, or even partially, underneath the template or blast marker. Remember that a model’s base is counted as being part of the model itself, so all a template or blast marker has to do to cause a hit is to cover any part of the target’s base.
So a blast can generate hits on a unit, but never rolls to hit.
RAI if you do not hit a model you missed.
RAW you miss if you scatter off table.