Interesting question, I'd never given it much thought myself. To the best of my knowledge, there is no "codification" of the microtonal intervals. After the human voice, one of the most popular (if not THE most popular) blues instruments is the guitar. Guitarists will "bend" the strings by pushing or pulling them across the length of the frets to increase tension, raising the pitch. How far they go is a matter of individual taste, some love to raise notes over the next half step, others just nudge it a little. Pushing & pulling along the string length also allows increasing/decreasing tension so notes can be altered. Of course, blues was probably the origin of slide guitar. Just thinking about it, I think sliding the fourth to the fifth is common, and bending the third is common. I'll have to play around on my guitar some, and see which notes I like to bend most frequently.

Frank i.t.s.