Hi Dal,I'm not sure about that black box on TV, it could be TV internal issue and factory reset may or may not help to remove it or the black box might be coming with the video device connected to the TV.To figure out if the black box is generated by the TV, turn Off/ disconnect video source from TV and if black box is still there then try the factory reset as mentioned in previous reply, since original TV remote is missing then I would request to use control buttons on the TV is Setup button isn't available on replacement remote.If factory reset doesn't work to remove the black box then please reply back to find out what is causing it to appear on TV screen.
You would need to go through first time setup screen on TV after performing factory reset/ shipping condition.Thank you.
Maybe the black box is PIP ? Can this be removed manually ?
Yes, if the TV has PIP feature then please find PIP/ Multi PIP button on TV remote and press it to cancel the PIP feature that should remove the black box on TV screen.