In $ \Lambda $CDM cosmology neutrinos start free-streaming after weak decoupling epoch and contribute to the damping of Primordial gravitational waves entering the horizon. New interactions can stop neutrinos from free-streaming, therefore, enhance the primordial gravitational waves at small scale. This leads to enhancement of CMB - B mode compared to $ \Lambda $CDM at higher multipoles ($ \ell $). This enhancement mimics blue initial spectrum of primordial gravitational waves. Therefore departure from the scale invariance, if observed in future observations of CMB - B modes, can be a signature of non-standard neutrino interactions. High precision detection of primordial gravitational waves or CMB - B modes will be able to distinguish between blue initial spectrum and new neutrino interactions.