Low-frequency Raman spectra of a diblock copolymer of polystyrene and polyisoprene (PS-PI) and a polystyrene-polybutadiene blend (PS-PB) have been studied. Low-frequency features, viz,, the quasi-elastic lines, Boson peaks, and vibrational lines, if present, have been fitted to known theoretical expressions. It has been found for the first time that, in the diblock, the Boson peaks manifest themselves at frequencies characteristic for homopolymers; that is, big enough regions formed by pure components exist in the diblock. The temperature dependence of the overall integrated intensity in the low-frequency region undergoes a break at the order-disorder transition temperature of the PS-PI diblock and manifests a marked nonlinearity near the temperature of macrophase separation of the PS-PB blend.