For three seconds, D.Va removes enemy projectiles in a 15-meter cylindrical range in front of her. It also protects her allies. She cannot attack during this time, but she can still move.
Does this mean that if Reaper uses his Death Blossum while inside the cylindrical range, every single shot would be blocked? Even the ones directed in the direction opposite D.Va (the lazer would have to go through Reaper and not have LoS)?
Yes, that’s exactly what it means. Incidentally, that’s an easy way to unlock the Shot Down achievement, which requires you to prevent 1500 damage in one use of Defense Matrix.