Does one get Japanese citizenship by "right of blood"?

jus sanguinis ≠ "from/by blood"

There are two primary ways to get "natural born" citizenship in most democratic countries: "jus sanguinis" (right by blood) and "jus soli" (right by soil). Some countries (not Japan) also given citizenship/nationality through marriage — jus matrimonium — or through religion.

Confusion arises from the term "jus sanguinis" because anybody who has studied Latin knows that sanguinis means "blood". A less confusing way of understanding jus sanguinis would be to not think of it as meaning "by blood", but rather, "by a birth parent's legal nationality". "By blood" often makes people think of race. This misunderstanding is further perpetuated by the present reality of demographics in that Japanese, even those who are naturalized, are usually racially Asian (CJK), and by the fact that jus soli isn't common outside the "new world" continents of North and South America.

To give an example using race (and avoiding issues of jus soli) intentionally to make a point:

If a black Nigerian national marries another black Nigerian national, and either one (male or female) of them naturalizes (changes their nationality) to Japan, then flies back home to Nigeria to give birth, that child will be a natural-born† Japanese citizen that could some day in theory become Prime Minister.

To give a counter example to show how jus sanguinis does not apply strictly to bloodlines:

Suppose a Japanese national U.S. resident alien (an 一世 {issei}, or "first generation"), marries another Japanese national U.S. resident alien 一世 {issei}, then they give birth in the United States. That dual-national child, upon turning 22, decides to select United States citizenship and gives up their Japanese citizenship. That Japanese-American meets and marries another Japanese-American in the same scenario: he/she relinquished their natural-born Japanese citizenship at the age of 22. Even if they fly to Japan, get permanent residency, and then have a child, that child will not be a legal Japanese national/citizen and will instead be a natural-born† U.S. citizen born in Japan… even though both the child's parents (二世 {nisei} or "second generation") and grandparents — and beyond — are 100% "racially (blood) Japanese".

If that child should later decide to naturalize to Japanese, they will probably qualify for the "simplified naturalization" (簡易帰化 {kan'i kika}) track due to their not so distant generational connections with Japanese national family. Likewise, that child would also probably be eligible for a 定住資格 {teijū shikaku} (long term resident status) visa, due to the same reason.

The Japanese Nationality Law mentions birth parents and nationalities as variables used for the acquisition of citizenship, but does not mention race or blood anywhere, in either the rules for natural born citizens or naturalized citizens.

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† The term "natural-born citizen" is the opposite of "naturalized citizen"; it doesn't mean being born on the soil of the land. It means acquiring the citizenship automatically at birth, as opposed to naturalization, which is acquiring citizenship not automatically (voluntarily) after birth.