Just a quick suggestion regarding the balancing of Necromantic spells. One simple check might be to require a Concentration or Spellcraft roll to handle the extra Negative Energy during the 'daylight' hours. The DC is really a question of how you want Necromancy to feel in the game - the higher it is the more 'deranged' Necromancy becomes as a pursuit. But failing the roll inflicts damage on the caster (say 1d4 per spell level), as the Negative Energy devours his lifeforce. An alterntive might be some damage to attributes, probably Constitution, though Charisma might be intersting (as the Necromancer merges more with the ambient Negative Energy).
All this pre-supposes, btw, the standard D&D3 interpretation that Necromancy is purely a negative act, associated with pure destruction of the lifeforce. I mention this because a variant that went to the older definition that put Necromancy as the magic of manipulating the power of life and death would have an unusual spin in this type of setting. All healing becomes necromancy too, but it is not benefitted by the power of the Black Sun. That makes necromancers more grey than black-and-white, even though they all run the same risks. Perhaps even the positive aspects of necromancy are, however, to be mistrusted...