The problem with africa, well one amongst many and I'm going to generalise horribly here, is that the security situation is a mess: No adequate policing or legal recourse coupled with huge diparity between rich and poor. The recipe will always lead the same place. Dar isn't the worst but, you have to keep your wits about you wherever you are in the world.
If anyone tells you that any place in this continent is "safe" it just means that they didn't stick around it long enough for the honeymoon period to wear off.
That said there's many places in Europe I wouldn't want to walk alone at night. Coldharbour lane, Brixton anyone?!!