The word 'bled' is the past tense of the verb 'to bleed' (saigner). It is used here to perhaps describe the steady pace at which the darkness entered the scene.
'Outer reaches' refers to the furthest or most mysterious parts of something. For example 'the outer reaches of space' could be areas in space that we haven't discovered yet, and very far from where we currently are. "It was just after sunset and darkness bled in from the outer reaches of the sky" describes a scene where the sun is going down and darkness is slowly taking over, as night time begins.