What is the instrumental answer (call and response principle) or continuation of the blues lyric?

If I understand your question correctly, it's this.

Listen to Robert Johnson. Take Crossroad Blues as an example. The vocal part sings, "I went to the crossroad, fell down on my knees.' Then the guitar answers, and the vocal part is repeated, followed by the response of the guitar.

It's called a Rif and it starts and ends with the root.

Watch this YouTube video:

http://youtube.com/watch?v=qTKgK70nhJ4

There are two examples of call and response here. In Muddy's intro (isn't he so stylish and awesome?), he starts off with the free-form "ohhh yeah" thing, in which he sings a line and the guitar player answers with something. They go and back forth. Muddy calls and the guitar responds; they interact with each other, one at a time, answering melodies with melodies.

Then the proper song begins. Muddy sings each line and the band responds to what he says with the same riff; Muddy's line changes constantly and the riff always stays the same. This is also call and response. I think this is interesting because I think it's debatable which part you'd consider the call and which is the response--notice that the band actually starts, and Muddy's lines are a response to the band's static riff.

This song, by the way, is considered the CLASSIC example of call and response in the blues.