The nano-structured materials possess higher band gap than its bulk form. You can find the mathematical derivation in any higher level book of nano physics. The augmentation in the band gap energy introduces the optical blue shift.
Have in mind that the absorption band edge (I mean actually the transmittance edge) depends strongly on the thickness of the film. To be sure that there is no thickness effect I will suggest to calculate absorption coefficient and optical band gap. May be you have already done this. What is the difference in optical band gap values?