This is a repeatable issue. I'm setting up all new title blocks in Revit this month and using CAD files as the base. Without fail, Revit will not properly import CAD files into a title block family. I've been through this fiasco 4 times already. CAD files were edited in AutoCAD 2018 (most current version as of this post) and imported (the only option for working with a title block family - so linking is not possible.) No matter what options are selected (and I methodically tried them all one afternoon) Revit completes the import but there is no CAD data visible. Visibility Graphics is correct, import scale is correct, nothing is hidden or turned off. I know the data is there because the Imported Categories tab in VG will expand to show me every layer in the CAD file that was supposed to be imported. I can even Undo the command "Import Vector Data" if I choose to. More annoying is that if you poke around the view you can actually grab the invisible CAD file and see its bounding box. It will show up in properties as the file you imported. The only way I was able to make the issue go away, was to copy all the CAD data in AutoCAD and paste it into a new AutoCAD drawing as above. Then save that drawing and import that into Revit.
The only clue I had as to why this is happening is a short error Revit gives before the CAD data is successfully imported: Revit complains that "Imprt detected no valid elements in the file's paper space," and asks if I would like to import data from model space. If I answer yes, then the CAD file will be successfully imported. Before copying and pasting into a new file, I did not get the paper space error. So I checked the original CAD file... and sure enough the paper space had a viewport in it. I erased the viewport and attempted to import it again and it worked! Revit doesn't ask whether you want to import from Paper or Model space in a CAD file and I did not expect Paper Space to be the default option. This may also be unique to Titleblock families which may assume paper space is the way to go more than other families. Add "checking the paper space for valid elements" to the list of things to check in the CAD file that won't import properly. I haven't tested other family types for the error yet and I'm not sure what logic makes Revit think that Paper Space is right place to import from without asking...