The Blue Suns were the most powerful mercenary group in the Terminus Systems and recurring enemies in Mass Effect 2.
The Blue Suns were co-founded by Zaeed Massani and Vido Santiago , with the latter managing bookkeeping while the former led the men into action. However, when Vido attempted to hire Batarians as "cheaper labor" Zaeed opposed the decision on the grounds that the Batarians were terrorists. This proved to be just the first of many incidents of Zaeed opposing suggestions put forth by Vido, and eventually Vido decided that Zaeed had to go. Betraying him and shooting him in the face, Vido seized control of the Blue Suns for himself.
Afterwards the Blue Suns would get progressively more cruel (at least according to a still-living Zaeed) and began dabbling in everything from slave-trading and slave labor to outright terrorism. Blue Suns cells were present all throughout the Terminus Systems, where they performed all manner of crimes and misdeeds. During his/her fight to stop the Collectors, Commander Shepard wiped out several Blue Suns cells and crippled many of their operations throughout the Terminus Systems, though it's unclear just how much damage this did to them as a whole.
Though still very much amoral and self-interested, when the Reapers appeared the Blue Suns recognized the threat they posed and agreed to enter a temporary alliance with the rest of the galaxy against the greater threat. It is unknown what happened to them after this, or even if the organization survived to the end of the Reaper War.