I just bought a box of Slickrock paper which looks great, but my first print has two small pure white blemishes. It looks as though the paper did not take the ink. I'm using Epson's pigmented ink, K3 series with an R3000 on 13x19 paper. I've printed this exact image on other paper, so I do not believe that it is the image.

What do you recommend to retouch these spots?

Most likely those spots are flakes, where the coating comes off after the ink is laid down. While you can retouch it by using a bit of the ink in your printer's cartridges, it's generally easier just to reprint the image.
To avoid flaking in the future, it can help to riffle the paper before loading it into the paper tray. If it is a problematic stock, sometimes brushing each sheet with a drafting brush or antistatic brush will dislodge errant flakes before printing.
To retouch, you need to open used cartridges, mix the ink and spot the print. Another method to try if you don't have empty carts: print a small section of the image (the section that was damaged) on a sheet of acetate. Carefully, because the ink will not dry. Then use the ink in that section to spot with.
Again, generally easier to reprint, and to avoid the problem in the first place. Many times, it's the first couple sheets or the beginning and end of a roll that have the worst flaking issues.