The closest you could get would be to describe the hair color:
* Laʻi (lah-ee): Means "pale" or "light," and could be used to describe light hair, but it's not specific to blonde.
* 'Ōlena (oh-leh-nah): Means "turmeric," and is sometimes used to describe yellow hair, as turmeric is a yellow spice.
* Heleʻele (heh-leh-eh-leh): Means "yellow" in general, and could be used to describe blonde hair, but it's not as specific as "blonde."
You could also use the English word "blonde" as it is commonly understood in Hawaii.