we use for edit pdf drawings Adobe Pro. For safety we must remove for example the customers name brand on the pdf dokument. We use the function : blacking.
After push the buttom: blacking operate...the text or picture is removed from origin. We save the document in our system.
Q: can i be really sure, that nobody (or a software tool) can rebuilt ( backing up ) this "removed area"?
The only safe way of removing information from a PDF permanently is by using the Redaction tool.
I don't know what you mean by "blacking", but if you're just placing a black comment or field over it then NO, your information is not removed and can be easily retrieved.