Documentation of 不+"x" contractions resulting in "r" sounding initial consonant?
This phenomenon is a non-standard regional accent, known as 儿化音. It is a phenomenon in some part of North China, especially in Beijing.


  不知道 (bù zhī dào) → 不儿道 (bùr rào)
  
  不是 (bù shì) → bú shì → 不儿 (búr)
  
  告诉你 (gào sù nǐ) → 告儿你 (gàor nǐ)