Final answer:
An a cappella choir sings without instrumental accompaniment, performing both religious and secular music with arrangements that may mimic the sounds of instruments.
Step-by-step explanation:
The vocal group that sings a song like Dark Eyes without instrumental accompaniment is referred to as an a cappella choir. This style of singing, which can be traced back to religious and folk music traditions, relies solely on the voices of the choir members to convey melody, rhythm, and harmony. Often found in both spiritual and secular music, a cappella performances can range from simple melodies to complex arrangements that imitate the sounds of instruments.