Board Thread:Naruto Discussions/@comment-26993711-20160319001508/@comment-26773508-20160320034903

@Seelentau: Perhaps Obito activated Izanagi the moment he saw Sasuke's eye change to Itachi's MS. There's almost always some time gap between moment when Itachi/Sasuke activates the technique and the moment when the black flames are actually manifested onto the victim. Sensors like Nagato could figure out Itachi activating the technique long before the black flames were actually produced.

So I think Obito had sufficient time to activate Izanagi as the sequence of events gave him the necessary time. First, Sasuke's eye suddenly changes to Itachi's MS pattern. Then, blood starts flowing through the eye. And only after that do the black flames consume Obito. I think that's sufficient time for Obito to react with Izanagi before the flames hit him.