How can I type special characters? [duplicate]

You need the unicode hex values for these as they aren't part of the standard Compose Key lineup. To work out what the keys were, I copied and pasted the characters off your question and stuck them in this converter. There are many others like it. It gave me the following:

U+25A1 : WHITE SQUARE
U+25A0 : BLACK SQUARE

To enter these, you follow the standard Unicode entry sequence:

Control + Shift + U, unicode hex (in lower case), Return