Adobe Acrobat allows you to use the Commenting tools to highlight text in a document. When you highlight text, the default color is yellow, however you can change the color to match your preferences. If you want to black out text, you can use black highlighting to obscure the text from view.
Click on the "Tools" menu, select "Comment & Markup" and select "Highlight Text Tool."
Click and drag the cursor over text you want to black out. It automatically appears highlighted in the default yellow.
Right-click or control-click on the highlighted text and select "Properties."
Click the color swatch and select the black swatch from the options. Set the "Opacity" to "100%."
Click the "Make Properties Default" to set black as the default highlight color, if desired. If you do not do this, you will have to adjust the Properties every time you highlight text. Press "OK" to leave the "Properties" window. The highlighted text appears blacked out.