I am not an expert, but my only suggestion is to make sure that whatever method you use is indeed permanent, so that someone with technical knowledge cannot merely use an editing program to remove the black bars.
It reminds me of several high level government cases lately where a document was created using MS Word and the passages intended to be confidential were covered with thick black bars. The document was then converted to a PDF, to prevent readers from viewing the information below. What the U.S. Attorneys did not realize, apparently, was that readers could use the text selection tool in any PDF viewer to copy the text beneath the bars. The readers could then paste the text into any word processor and read what the government had wanted to keep secret. One journalist used Adobe Acrobat's "Touch Up Object" tool to select the black bars covering the text. He then hit the delete button. The black bars disappeared, leaving just the text. Almost humorous, but I guess the consequences could be serious.
The moral of the story is that there are proper ways to add those black bars. I think it's called redaction. If you do a Google search under "redact" or "redaction", there will be a gazillion hits, including for videos.
Good luck. I hope it helps.
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