Do all people with non-Japanese blood have to naturalize to become legally Japanese?

Some people mistakenly believe that the only way to get Japanese nationality at birth is from a racially/ethnically Japanese parent, but this is not true. There are many ways to acquire Japanese nationality other than naturalization, but the most common method is by birth to a parent who already has Japanese nationality.

For example, if one or both* of a Caucasian/Black/Latino couple naturalize to Japanese, their future racially non-Japanese child will be Japanese at birth, automatically. Even if the child is not born in Japan ("jus soli"; right of the soil).

* naturalizing to Japan as an entire family unit is not unusual and there is a procedure for it.

It's not uncommon, in fact, for children born in the United States to receive Japanese nationality due to one of the parents possessing Japanese nationality. In most of these cases, one of those parents is ethnically/racially Japanese — the child's racial makeup is often described as ハーフ {hāfu} (multi-racial Japanese) — so some people mistakenly think that racial lineage is the criteria for citizenship at birth. In most countries of the world, though, the concept of jus sanguis ("right of blood") does not mean blood as in race; a better way to think of jus sanguis is "right of parent's legal nationality".