Here is how you do it:
- Go to the Sheet view
- Make some text
- Set the text Type to Opaque and make the color just one very small step above pure white (253-253-253), otherwise Revit will make your 'white' look like 'black'
- Draw some full stops (periods) to fill in the text box
- Put it on top of what you want to mask
There is another method that is slightly more involved - you can put a shared Generic Annotation family with a Masking Region into a Drafting View and put that on the sheet. Check out this link, where I reposted this method from The Revit Clinic.