Use the Free Select Tool in Addition mode (after your first selection, hit the Enter key, then select the second part)
EDIT: Ooppss my bad, I did not see that you wanted black (see previous answer after the horizontal line, in case it can help a visitor to just desaturate the red)
Then go to Colors > Desaturate > Mono Mixer... diminish the red channel to get it black as your liking
Or... Colors > Desaturate > Desaturate in Luma mode
PREVIOUS answer (just to desaturate the red)
Then go to Colors > Hue-Chroma..., put the Chroma slider at -100 (minus 100), and you're done.
Or another filter/method, go to Colors > Hue-Saturation put the Saturation slider at -100 (minus 100) and diminish a bit the Lightness` slider to your liking, and you're done.
EDIT: Ooppss my bad, I did not see that you wanted black (see previous answer after the horizontal line, in case it can help a visitor to just desaturate the red)
Then go to Colors > Desaturate > Mono Mixer... diminish the red channel to get it black as your liking
Or... Colors > Desaturate > Desaturate in Luma mode
PREVIOUS answer (just to desaturate the red)
Then go to Colors > Hue-Chroma..., put the Chroma slider at -100 (minus 100), and you're done.
Or another filter/method, go to Colors > Hue-Saturation put the Saturation slider at -100 (minus 100) and diminish a bit the Lightness` slider to your liking, and you're done.
Patrice