Suppose that someone guessed something correctly, but it's totally a guess, he knew nothing about it. What do you call this guess? Is it natural to say a blind guess?
"Blind guess" is fine. Other words that can be used are
Stab in the dark: (The Free Dictionary by FARLEX) proverb A guess or estimate with very little or no assurance as to its accuracy; a wild guess.
Shot in the dark (The Free Dictionary by FARLEX) A guess or estimate with very little or no assurance as to its accuracy.
Wild guess (Wiktionary) A guess that is not based on any evidence, knowledge, or experience.
If we are focusing on the result "fluke" can also be used .
Fluke (Wiktionary) A lucky or improbable occurrence, with the implication that the occurrence could not be repeated.
A "blind guess" seems idiomatic and fine to me, you also could call it "a shot in the dark":
an attempt to guess something when you have no information or knowledge about the subject and therefore cannot possibly know what the answer is