Apart from the odd mistake everyone blacked out should be someone who might still be alive (b1920 and after). Obviously from other records you can surmise who they are (children of householders usually) but you do come across ones you can't account for - 4 blanks but only 3 known children for example. These could be other relatives, grandchildren, nephews, nieces etc, or children they're looking after. Also I think it's been mentioned that the evacuation of children had already begun.
I think to have someone un-blacked out you have to supply a death certificate but they also have to be at the address you think. If they're not there I don't think they'll look for them.
I found a relative - a young woman, aged 20 - living in London with another young woman. Also there is a blacked out record and it made me wonder if she had a child but it could also be another young woman who is a little younger.