Traffic grooming in Wavelength Division Multiplexing (WDM) optical networks is a promising technology for improving the performance and reducing the network cost. We study the blocking performance of a single-hop WDM traffic groomed optical network, by considering that different connections request service through a single optical link. In each connection the generated calls may come from a single or multiple service-classes. For the case of a single service-class we develop a novel analytical loss model to calculate Connection Blocking Probabilities (ConBP) and Call Blocking Probabilities (CBP). Also, we extend this model to support multiple service-classes. The accuracy of the calculations is verified by simulation and found to be absolutely satisfactory.