Azoindoxyl

For example, certain sulfur-containing dyes reduce osmium tetroxide to osmium black, which is electron opaque (Hanker et al., 1964), and certain azoindoxyls form insoluble osmium-containing complexes ( Holt and Hicks, 1966).

Source: wiktionary

The nitro-blue tetrazolium method with menadiol diphosphate was found to be superior to existing methods employing azo, azoindoxyl or tetrazolium salts and to metal precipitation methods.

Source: wiktionary