1. Composition for dyeing keratin fibres, comprising a physiologically acceptable aqueous medium, comprising:
at least one dye bearing at least one ionic group, said dye being present in an amount ranging from 0.001 to 5% by weight relative to the total weight of the composition;
at least one block copolymer comprising at least one water-soluble nonionic block and at least one ionic block exhibiting charges opposite to that of the ionic charge borne by the dye.