When two different tones blend so well when sounded together that they almost seem to merge into one tone, the interval is called a(n)?

Answerbarrel vault

Artists usually use musical imagery to make their compositions more unique and alive. imagery in music used to "show" music. because of that listeners feel what happens. they see music like complete composition. they hear sounds, they feel movement, they see a picture. it makes music deep. that's why  musical imagery is very important and is used by artists so frequently.