From this John Wickham tweet:

Don't use the Markup highlighter on your iPhone to censor information in screenshots. Multiple strokes don’t make it opaque, and you can really easily reveal what’s beneath it. pic.twitter.com/Gm43aYz0LY

— John Wickham (@thejohnwickham) June 20, 2018

Check the pic on the left and on the right. See the difference in the blocked text?

My followup to John asked how he got from one to the other. In other words, how did he reveal what’s under the marker.

His response:

In the Photos app, tap Edit, then open the levels (dial icon in the toolbar). Tap the disclosure arrow for Light, then crank the Brightness slider all the way up.

Yup. This is really good to know. I believe a filled rectangle will work. Nice find, John!