When refilling Canon ink cartridges why do you have to completely seal the black ? -

and the color you only have to replace the sticker?(if there s anything left) like shred it trying to lift it, like l did.

Air bubbles or some such thing is supposed to get in and will cause problems with the ink flow. I have to confess I never seal either my colour or black when I refill them and to date I ve never had a problem. If you do want to seal it just use sticky tape over the hole.