A reactive metal formazan blue dye represented by a free acid of the formula, ##STR1## wherein A.sub.1 and A.sub.2 are independently a substituted or unsubstituted phenylene or naphthylene group, B is a straight or branched alkyl or alkenyl group, a phenyl group, a naphthyl group or a heterocyclic group, the alkyl, alkenyl, phenyl, napthyl and heterocyclic group being substituted or unsubstituted, Me is an ion of metal identified by an atomic number of 27 to 29, X is --O-- or ##STR2## Y is a halogen atom, Z is a group capable of imparting water solubility, each of l, m and n is 0 or an integer of 1 to 3, provided that the sum of l, m and n is from 1 to 4, D is a substituted or unsubstituted phenylene or naphthylene group, R is a substituted or unsubstituted lower alkyl group, and Q is --SO.sub.2 CH.dbd.CH.sub.2 or --SO.sub.2 CH.sub.2 CH.sub.2 L in which L is a group capable of being splitted by the action of alkali, which is useful for dyeing hydroxy- or amido-containing fiber materials with good build-up and level-dyeing properties to obtain a dyed product of deep blue color having excellent fastness, particularly against light, perspiration-light and chlorine.