You are right. The protein will be expressed at higher levels upon adding IPTG, but there is no reporter to assay for blue/white colonies

pET28a vector transformed in a bacteria confers resistance to kanamycin so you can grow the transformed bacteria in kanamycin and check for the successful transformation. Later on you can induce the expression using IPTG and could obtain large amount of the cloned protein.

There is LacI not LacO in the plasmid pET28a if I am not wrong. Please correct me if I am wrong that there would be white colonies and not Blue ones.

Deepika Nag : the vector carries a LacO operator between the T7 promoter and the 5'-UTR: this is where the LacI repressor protein will bind to block transcription, unless IPTG is added. You can express large amount of the transcript ONLY if the strain you use carries the T7 RNA polymerase gene, like BL21(DE3). The peptide encoded at the multiple cloning site, in the frame of which you clone your protein of interest, is NOT the LacZ alpha-peptide. Thus, the pET vector itself will not generate a blue color when expressed in a LacZ-delta M15 strain like the usual cloning strains. So blue/white selection is not possible.