Teleport was moved because Vengeance became level 4 as Shield replaced it in the Tier 5.

The idea was dark to be leader in mind control as in the original game. Berserk + Blindness do just that.

If I have to be fair Might and Magic franchise is so inconsistent across the series that you can take any spell and put it anywhere and it will still fits with some minor change in names.

Have in mind that Teleport is still available to Light school through the "Teleport assault" perk.

Another reason why teleport was moved to dark is that early game dark is very inefficient because it does debuff on units the hero cannot kill anyway or do damage-per-second that is too low to affect the battle at the stage of hero development. Teleport comes as a saving grace because you know that at magic guild level 3 you will either get teleport or sorrow, (Sorrow reduces creature luck and morale by 9) both of which will boost hero performance significantly.