Basil once it flowers tends to produce a more bitter taste in the leaves. Picning off the flowers is recommended unless you are specifically looking to seed.
In our household we remove about 2/3rds of the plant leaving just enough for regenration. this gives us an abundance of basil (quick grower) and elongates the growing period.
As a secondary comment it is worth growing new seeds now so that you will start to crop in the next 4-6 weeks. this continues your harvest but allows you to let others go to seed. Worth while mentioning, don't let them drop seed. Wait for the flowers to to bloom and seed. Once the seed pod is a fibrous brown collect them. not all will germinate but the majority will, thus saving you $3 per species.
We have some black opal basil that seems to have a gentic mutation going on. The opal is green and not purple, although it has purple spots running through the plant. We have decided to collect the seeds and run with them to try and collect the taste and perhaps name a new variety. (Although I'm sure that other people have non-purple black opal we are calling it Inamon basil!, or perhaps Bulglish basil (Bulgarian-English)