This a petunia! Lucky lucky you! This just might be a trailing petunia that stays low and works great in hanging baskets. My advice? Snip those flowers off as soon as you are able. With each flower just know there will be 10 or 20 behind it because you cut that flower off. These are annuals.
Annuals have but ONE job in this life and that is to produce seed. They are serious about this job. If you keep cutting of the seed babies the flowers then the plant gets even more vigorous, larger produces massive amounts of flowers to offset what ever troll is killing its babies.
The more flowers you cut off the more that plant will get energized and grow bigger, stronger and produce many more flowers than had you...made the mistake most do and let the flower stay, and go to seed.
Once that flower is allowed to make seed that plant is done. Well, the seed is signals to the seed counters (humanized) and as long as that plant knows it has produced some seed it will just start dying slowly and surely. It has done its job in life.
A little balanced fertilizer would do wonders. Have you added any fertilizer? Too much nitrogen and you'll get lots of leaves few flowers.
It's a petunia (cultivar Blue Morn? Starry Night?) - an annual - and will continue blooming throughout the growing season if you remove the spent blooms ("deadheading"). To have more of this plant for next year, you may want to stop deadheading the plant in late August and then let the seeds fall on the ground beneath it.