Word heading number blacked out

1. Place the cursor just to the right of the black box in any example of the affected heading.
2. Hit ← (left arrow) and the black box should turn gray.
3. Type Ctrl+Shift+S to bring up a style popup.
4. Click Reapply.

This has worked every time I've had this problem.

I solved this after all, I also noticed that someone else was unable to resolve this on a Microsoft forum, so if you read this, here is my solution:

1. Press properties for Heading 1
2. Press format, numbering
3. Chose No numbering. Exit the properties. All numbering will now disappear.
4. From the "start" pane, press multi-level-list, and then the format that looks like:

1. Heading 1

1.1. Heading 2

1.1.1. Heading 3

I tried all of the answers above, some worked, but... the black rectangle came back each time I reopened the document.

In short : I had to remove all the numberings (A) and recreate the multilevel list(B).

Here is how to do it:

A: Remove the numbering

1. Place the cursor on the first Heading 1.
2. Click on the button Numbering in Ribbon/Home/Paragraph to remove the number (1 on the picture).
3. Select the whole line where your Heading 1
4. Right-Click on Heading 1 in the Ribbon/Home/Style and select "update heading 1 to match selection" (2 on the picture)

Redo step 1 to 4 for all your headings (I had to go until Heading 6). Make sure by scrolling down in the navigation/heading panel, that all the number before your headings are removed.

B: recreate a multilevel list

1. Place the cursor just before the first heading 1
2. Click on Multilevel list in Ribbon/Home/Paragraph and choose the format you want (3 on the picture). [In a comment, fl0w also suggests to select "Define new multilevel list" and construct a multilevel list]