This invention provides a seal for the tank/developer section of a two-piece toner cartridge that is applied externally to the section at the opening of the developer roller, where it is intended to confront the drum in the fully assembled cartridge. The seal assembly includes a resilient seal member, illustratively constructed from a stiffening bar that caps a resilient/pliable foam or another elastomer. The foam impinges on the doctor blade where it meets the developer roller, to bias the doctor blade against the developer roller and seal this region against the exit of toner. The foam and stiffening bar have an elongated length that matches the developer opening so that this region is fully sealed along its length. The stiffener is flexibly attached to a sheet material. The sheet is secured to the cartridge surface at a respective edge on each side by releasable adhesive.