Pomegranate takes 3 -5 years to be mature enough to produce flowers and fruit and is a shrub/tree plant.
There can often be stray seeds in soil; even bagged soil could contain seeds.
If the pots are outside, seeds could be deposited by critters.
Sure looks like a Four O'clock plant and bloom. Wait until the seeds mature and you will know for certain; Mirabilis jalapa seeds have a distinctive shape.
