I feel like this is a fairly basic question, but i haven't found any real information online. how can i set the blending state to additive for a cesium particle system for an effect such as fire, etc?
Unfortunately it's not available in the current version.
Particle systems in Cesium are implemented as collections of Billboards, and currently the Billboard system is hard-coded to always use alpha blending. It would be a great feature to add though, but it would take some plumbing work to make an option for additive-blended billboards, and then another option for additive-blended particle systems that call for additive billboards. Both would be useful in their own right.