The term Blues has roots that predate the music we know as 'Blues' by at
least two centuries. In the time of William Shakespeare [1564-1616],
people believed in fairies and sprites---invisible, magical beings that
could assist or worry people as they went about their daily lives. In the
context of this superstition, when someone became irritable or depressed
for no obvious external reason, people would say that "the blue fairies",
or "the blues" were bothering them, like a cloud of gnats or mosquitoes
might worry us today.
By Thomas Jefferson's time [1743-1826], the notion of invisible fairies had
fallen into disrepute. But the expression of 'having the blues' can be
found in Jefferson's writings, and was still a common expression for those
less-than-happy emotions to which we are all prone.
Blues and other African-American art forms often show their deep African
roots in what are known as African Retentions---parts of African traditions
that we still find embedded in American and African-American music, art and culture.
In Blues, the easiest of these to identify include:
Call & Response: a 'conversation' in music between a solo 'call' and a group or instrumental 'response'.
Syncopation: a musical term for stresses that fall off the established beat.
Emotional Singing Style: which can include shouting, crying, screaming and other speech sounds not typically found in European singing prior to the 1950's and 60's, when African-based vocal styles began to be heard more widely.