Sounds like a useful feature. The question is.. would the command become a direct subcommand of flask such as flask doblueprintstuff or be somehow separated, e.g. flask myblueprint doblueprintstuff. I could imagine arguments for both cases depending on the applications.
Maybe blueprint.cli.command and blueprint.app_cli.command similar to how blueprints can modify some things (e.g. error handlers) either globally for the whole app or just for the blueprint.
Having both blueprint.app_cli and blueprint.cli seems not ideal to me. Also, you should have a way to specific the subcommand group a command falls under. Default that to the blueprint name, but allow an override.