Yeah, but if Merlin left any blood descendants at all, by now there would almost certainly be Merlinian blood in half the population.
Genealogy is funny that way...
Not all bloodlines do; some don't reproduce well, they just... dwindle, and end.
I'm pretty sure the Archive bloodline has major mojo associated that ensures there's always one daughter -- but never more than one -- to receive the Archive itself when the prior holder dies.
I'd be unsurprised to learn that there was some weirdness around Merlin's bloodline; but it could equally-well be some working where the OG Merlin made sure that literally everybody in the world was his descendant (maybe his blood had some uber-Starborn world-protection properties... or maybe he was just terribly egocentric that way); or maybe he ensured that he had
no descendants, because his bloodline was too dangerous.
I don't think we've seen either canonical in-universe nor WoJ on the matter; only that Eb (and thus Harry) are in the Merlin's Master-to-Apprentice
teaching lineage.