Blobbies are just the RIB representation. In the renderer it builds a 3D microvoxel grid (like Mantra does) in much the same way as 2D grids work in normal REYES. This also means that intersecting volumes are handled by the renderer without you caring about it, and it has shading rate controls etc.
3delight does have some API which is supposed to be used for volumes, but I have never used it and it does’t look particularly easy to use. It also doesn’t come working out of the box:
http://www.3delight.com/en/uploads/docs/3delight/3delight_73.html#SEC283
Simon