As you said the black rectangle is simply a visual indicator, so you need to give that screen reader something to see. The simplest way is to give an alternative text such as "redacted" or "this text has been redacted by person on date" whatever information it is you can give, then black out that text by changing the color visually using CSS. Like so: https://jsfiddle.net/su35fdcc/
<p> Area 51 really hides secrets about <span class="redacted">Redacted text</span> and other oddities.
Lands you with this:
This way, any sighted users simply see a black rectangle like you would expect. However, visually impaired users using the screen reader will come across the word and it will read "Redacted".
Tested with NVDA screen reader to make sure that it works as expected.