The presence of blue straggler stars (BSs) as secure members of Galactic open clusters (OCs) poses a major challenge to the conventional picture of evolutionary population synthesis (EPS) based on the stellar evolution theory of single stars, since the major formation ways of BSs are all correlated with stellar interactions. With a working sample including 100 Galactic OCs with age ranging from 0.1 to 10 Gyr, the contributions of BSs to the integrated light of their host clusters are calculated on an individual cluster base. We also show in this work that the intrinsic evolutionary stages of OCs can be enormously misunderstood by the conventional simple stellar population (SSP) models, if the real/observed integrated spectral energy distributions (ISEDs) of OCs are the only thing that we can depend on and are fitted with ISEDs of conventional SSPs, and consequently the huge uncertainties in age and/or metallicity can be conservatively estimated at 50%. Thus, we strongly confirm that when the conventional EPS technique is used to study the properties of stellar populations in complicated systems such as galaxies, the contributions of BSs should be definitely taken into account in order to avoid big fitting mistake.