A Blapper is an unofficial term used by City of Heroes players to describe a character that attempts to merge the characteristics of a ranged "Blaster" archetype with those of a "Scrapper" melee fighter.
Although an "intermediate" archetype already existed within the MMO in the form of the "Defender", some players believed that the available powersets were not sufficient to create the characters they wanted to use. This led to a series of unofficial "mash-up" archetypes, with the "Blapper" being the most popular of them.
Paragon Studios were in the process of developing a new intermediate archetype called Sentinel that was in the beta testing phase and in plans to being released with Issue 24. Unfortunately it was not made available due to the MMO's forced shutdown.
(Please see the Paragon Wiki entry on the subject.)
The hero Kinetic Kitten is considered a prime example of a "Blapper".