A reactive black dye composition is disclosed, which comprises: (A) a reactive blue dye represented by the following formula (I) or a salt thereof; and (B) a reactive red dye or a salt thereof, a reactive yellow dye or a salt thereof, or a combination thereof.

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Herein, R.sub.11, X.sub.1, X.sub.2, X.sub.31, X.sub.41 and n are defined in the specification. In addition, a method for dying fibers using the aforesaid reactive black dye composition is also disclosed.

The present invention relates to the field of isomeric mixture of reactive dyes that are suitable for the dyeing or printing of nitrogen-containing or hydroxy-group-containing fibre materials and yield on such materials dyeings or prints having a good build-up behaviour, a high fixation rate, good washing off properties of the unfixed dyes and good all-round fastness properties, as well as a good stability to various dyeing parameters such as a good bath stability. Furthermore, the present invention relates to a process for dyeing or printing of nitrogen-containing or hydroxy-group-containing fibre materials, wherein the isomeric mixture of reactive dyes according to the invention is used. The isometric mixture of the present invention comprises at least an isomer of formula (Ia) and an isomer of formula (Ib). ##STR00001##