If a client has a document with text they want to keep yet not have visible in the final PDF version of the document, you do have some options.
One option would be to encourage presenting the material in a way which would not require blacking out text or images.
Another method would be to redact the material within the public version of the file which would use Acrobat's Redaction feature.
Redact is a tool available in Acrobat.
The procedure includes:
(1) Selecting the text and/or figures to be removed.
(2) The Redact tool includes a wizard which walks you through each step.
The result is a black bar where the items were located within the document. Note: PDF redaction removes the items (text or image) permanently from the PDF file.
You must add tags with your usual method. If the Acrobat AutoTag feature is used, the redacted material will appear as a Path within a Figure tag. You can artifact the Tag container and delete the Figure tag.
An item to keep in mind is the user may become confused with the blank within the flow of the document. You could keep the Figure tag with the Path container and add Alt text indicating, "Redacted".