The production of oxidation-stable and mechanically strong metal layers having a black color presents a particular challenge in the area of electrochemical finishing, especially since there are only a few metals which are suitable for this purpose. A possibility which is not hazardous to health, in contrast to nickel, and is economical compared with rhodium is the electrochemical production of black ruthenium layers. The invention provides an electrolyte and a method using this electrolyte for producing black ruthenium layers on pieces of jewelry, decorative goods, consumer goods and technical articles. The electrolyte is distinguished in that one or more phosphonic acid derivatives are used as a blackening additive. These maintain brightness. The degree of blackness of the resulting black ruthenium layer can be adjusted by the choice of the type and amount of the phosphonic acid derivatives used, while maintaining the desired brightness.