It looks like you meant RW420 printer instead of ZW420. In Zebra Setup Utilities -- Configuring Printer Settings article you can see how to use "Media Type" settings to control black mark detection and "Backfeed" settings to control backfeed behavior. Also, you can set respective settings value with "^MN","~JS" and ZPL commands.
I suppose following documents may be useful to you:
Software Engineer, Kutir Mobility
Posted on behalf of Zebra Technologies