The OP didn't mention the total quantities s/he is dealing with. That makes a difference from the point of view of local effluent regulations. Whatever treatment those regs suggest should be followed - not because they are the most factually or chemically correct, but because one therebye avoids a fine or imprisonment.

The oft quoted idea of mixing the blix and developer probably comes from the home-use version of the Cibachrome chemicals where this was designed in to the chemistry. It was not the same for the commercial version of the "same" process, from what I remember. In any case, unless instructions for the chemical products one is using specifically recommend this, it's probably wise to not try it out randomly.

The liquids are mostly water so, after de-silvering the blix if possible, can an animal and pet-safe evaporation of the water take place? Probably outdoors, away from home ventilation, not drinkable by passing creatures. That leaves you with a very small amount of solid(-ish) residue to deal with.