Finneas
teased Blood Harmony in August 2019. The EP marked Finneas' first solo release
after co-writing and producing material for his sister Billie Eilish for
several years. He expressed a desire for the "title to remain ambiguous to
listeners, and for everyone to be able to imbue it with their own meaning and
justification", but clarified that "blood harmony" is an
"expression for what it sounds like when siblings sing together, that
biologic chemistry — I think that speaks for itself".
The deluxe
edition of Blood Harmony contains two extra tracks. "Break My Heart
Again" is an indie pop ballad. The sound of typing and sending messages is
heard for the majority of the song, with it being almost entirely composed of
real texts between Finneas and a former lover. Lyrically, Finneas confesses to
being constantly hurt by the one he cares about. An alternative version of
"Let's Fall in Love for the Night", titled "Let's Fall in Love
for the Night (1964)", is a lullaby-influenced track. Critical
commentary noted the influence of Frank Sinatra.