I highlight the 6 cells. I click on the "Draw border grid'' icon and then choose 'Draw border' a bunch of dots show up elsewhere, a pencil icon, I try to draw using it and nothing happens. I try it a 2nd time and it works but I need to repeat the process somehow since it seems to want to do only one set at a time (1st just the outline, then the inner vertical ones and then the inner horizontal ones). Anyhow it worked. Thank you

Part of my original question remains though: Since the default (not mine but Excel's) seems to be blue lines, why do they disappear when you remerge? and why do they become black when you put them back manually? I find this ridiculous--but what do I know :-)