BLECE is a special bio-weapon project undertaken by S.I.N.
An acronym for "Boiling Liquid Expanding Cell Explosion", no-one is yet certain what the project entails, but it is believed that Seth organized a global fighting tournament in order to collect the data needed to complete it. Crimson Viper was sent to investigate under the guise of a S.I.N agent.
At the end of the tournament, Viper attempted to retrieve the data, but it was deleted by Cammy, who didn't want to let it fall into the wrong hands. It is believed that the key piece of data needed to complete it is the Satsui no Hado.
The project's status after the defeat of Seth is unknown.
Method of activation
It is shown in Street Fighter IV: The Ties That Bind that it allows a fighter to augment their natural ki energy, and requires ki-like power to activate; this is the reason S.I.N. hunts down Ryu and other martial artists, as their ki mastery made them the perfect test subject. A notable example of this is Ryu's Satsui no Hado, the aforementioned critical component of BLECE.
When activated, the subject's ki energy is artificially extracted and amplified. The resulting energy pulse resonates within the subject causing the mitochondria in the cells to overflow with excess kinetic energy. This in turn causes the boiling of moisture within living tissue which causes the cells to expand and detonate.
As a WMD
BLECE is a biological weapon by design, but Guile stated "It isn't chemical, biological or nuclear", meaning its true nature is unknown. It was the weapon responsible for the deaths of the animals in the Amazon seen in The Ties That Bind.