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I've finally got a decent solution, which is especially good for whiting out text:
1. Use Tools->Annotate->Text to create a text box.
2. Click in the box to get into the text entry mode.
3. Open up the Character Viewer (national flag in your menu bar, or option-cmd-T).
4. Double-click the "FULL BLOCK" character (the largest black square symbol) from the Geometric Shapes collection. This should put a black square in the text box.
5. Change the font size to get the desired height, if necessary, then
6. Copy the symbol, and paste repeatedly until you have the desired text blacked out.
7. Select all, change the font color to white, and admire your work.
NOTE: The original text is still there, hiding behind the white boxes, and can be uncovered by deleting the "whiteout" that you've just applied. To make the change permanent, use the print command, and under the PDF menu select "Save as PDF". The saved file won't contain the hidden text.