You could use the Area Hatch Fill in the menu Tools/Sketch Tools to fill objects with solid color. However, if you have text or shapes that have holes this will be difficult as they won't fill by selecting the inner and outer contours at the same time.
Even if you set the Area Hatch Fill to black in the sketch, when you exit the sketch it will show as the default grey. So you have to right click on the sketch in the Feature Tree and set the sketch color to black.
Then you could save as a JPG or TIFF. Make sure the you click the Option button when you save and change the deafault from "Screen Capture" to "Print Capture" to produce a higher resolution version suitable for engraving. Also select "Remove Background"
If you need to do a lot of text or shapes that have holes, I think it would be easier to use Photoshop, GIMP, Inkscape or similar program.
You could do it in SolidWorks faster by extruding the sketch. Then set the extrusion color to black and delete the lights. Save the JPG or TIFF as mentioned previously. NOTE that "Remove Background" does not work when saving the image from a 3D object. It results in a blank image. I'm not sure if this is a bug. You would then have to open the image in a graphics program and use the "Threshold" function to eliminate the background quickly.
Perfect! Thanks Chris, this is just what I needed!