Spectral data on 3/2 omega 0 emission from a long-scale-length carbon plasma produced by 1.064 mu m laser light are presented at an average laser intensity of 3.0*1013 W cm-2. Both wings of the spectrum in the backscattering direction, which were observed in the laser-plasma experiments reported earlier only at laser intensities greater than 2*1014 W cm-2, have been observed in the present experiment. The possible mechanisms producing the backscattered 3/2 omega 0 blue photons have been analysed. It is concluded that the specular reflection of the amplified blue plasmons excited by the convecting two-plasmon decay instability is density-scale-length dependent and is the effective mechanism for the observation of the blue wing in the spectrum.