Question
why are my birds of paradise the flower part turning black?

Answer
Kathryn,
Usually if a flower is turning black it is a fungal condition, often caused by frequent sprinkling with water.  If the flower could be getting damp from morning fog, a sprinkler system or hand watering, that could cause what you are seeing.

If that is not the case, it could be any of the following:  Fertlizer burn - too much fertlizer, or fertlizer when the plant is dry.  Or it could be another substance hitting the flower: a lawn product, a cleaning product used on a nearby house or windows, or a garden product (fungicide, herbicide, insecticide) that hits the flower on a hot sunny day.  

At this point, clip off the blackening flower, water the plant deeply less often (a deep soaking once every five to seven days is better than a little every day) and watch how the plant does from now on.

I hope this helps!
C.L.