The princess blouse was previously a reward from the Kiss the Frog random event. It can now be purchased for 2,000 coins from Iffie's Random Costume Shop in Varrock. Like all other random event items, it gives no stat bonuses. It can only be worn by female players; if a male player attempts to equip it, it will change into a prince tunic.
Before an update, a player could trade in a frog token for the entire princess outfit (the princess blouse and the princess skirt). After said update, players would be given a random event box, which could give the player a random piece of the outfit designated to the random event.