I've been looking for documentation showing 不+"x" contractions resulting in "r" sounding initial consonants, but to no avail.
This is an interesting phenomenon and I'm sure the regular person would say: "it's just people talking fast, don't over think it," but in technical terms these would be some form of contractions. Think:
不 + 知道 = bu rao
不 + 是 = bu ri
I'm interested to know what kind of research has been done on these contractions, if any.