[2] "3 O'Clock Blues" effectively launched King's career and remained a part of his concert repertoire throughout his life.

[4] According to music historian Ted Gioia, the song lyrics start out "as an insomniac's lament, but end up with a weepy farewell more suited to a suicide note":[5] Well now it's three o'clock in the morning, and I can't even close my eyes ... Goodbye everybody, I believe this is the end By the time of the record's release two years later in 1948, Fulson's style had already evolved into a West Coast blues style typified by his hit recordings for Downbeat and Swing Time Records, such as "Every Day I Have the Blues" and "Blue Shadows".

Nonetheless, writer Colin Escott notes that the song "clicked where the others hadn't [perhaps due to] the new found drama and urgency in B.B.

[9] King's version is a slow (65 beats per minute)[5] twelve-bar blues notated in 12/8 time in the key of C.[10] Blues historian Robert Palmer sees King's guitar work on the song as showing his T-Bone Walker influences, "though his tone was bigger and rounder and his phrasing somewhat heavier".

[12] King also used melisma, a vocal technique found in gospel music, in which he bends and stretches a single syllable into a melodic phrase.

[17] The induction statement described it as "the first record to amply capture the emerging brilliance of both his [King's] singing and guitar playing talents".