Well, first you need to understand the mood you want to convey for the story. If it's a sad story, a minor blues scale might be better. For a blues harmonica riff, focus on using the lower register of the harmonica as it often gives a deeper, more resonant sound. Experiment with different rhythms. A slow shuffle rhythm can be really effective for storytelling. Play around with long and short notes, for instance, a long drawn - out note followed by a series of short staccato notes to create contrast and keep the listener engaged.
One simple way is to start with a basic blues scale on the harmonica. For example, in the key of A, the blues scale is A - C - D - D# - E - G - A. Use slow, drawn - out notes to set a mood. You can also add some bends and vibrato to give it more emotion, like bending the third note (C) slightly downwards to create a more soulful sound. Try repeating a short sequence of these notes to form a riff that can carry the story in the music.