Blonde is signed by making your dominant 'Y'-shaped hand (i.e., the sign for 'Y' in ASL is a fist, with pinkie and thumb extended). Start with your thumb beside your head and shake your hand as you move your hand down from your head to your side. The sign looks like your are teasing your long blonde tresses.
Blonde is a good sign for advanced signers who are already confident with colors. You can teach them the signs for a variety of hair colors, and then look through pictures asking them to sign the right hair color. Hair color is a good advanced skill, because hair color is often a little ambiguous, and will help teach your baby how to find the closest match.
As well as teaching hair colors from real-life experience, you can also add the flash card to add a little variety to your baby’s signing. This practice of using both real life and flash cards helps teach your baby to make their understanding of the words more robust.