Upvote:4
TL;DR: David Patterson has confused the philosopher Democritus (of Thrace, 4th century BCE) with the historian Damocritus (mentioned in the Suda, probably 1st century BCE).
This is a simple matter of following citations. Patterson cites Trachtenberg, who says:
Probably the oldest report of a ritual-murder charge is from the pen of a pre-Christian writer, Democritus […]8
Joshua Trachtenberg (1943). The Devil and the Jews, p. 126. Yale University Press.
In endnote 8 on p. 243, Trachtenberg cites this claim to the following entry in Théodore Reinach:
Damocrite. Historien d’époque inconnue, mais probablement contemporain des Alexandrins Chærémon, Lysimaque, etc.
60. — Suidas, s.v. Δαμόκριτος (C. Müller, Frag. hist. graec., IV, 377)
Damocrite, historien. Ouvrages: Tactique en deux livres; Sur les Juifs
Théodore Reinach (1895). Textes d’auteurs grecs et romains relatifs au judaïsme, p. 121. Paris: Ernest Léroux.
It seems that Trachtenberg (or his typesetter) made a typographical error, and that Patterson neglected to check.