A Blu-ray drive is not a current option for the Lenovo W520. This deficiency can be alleviated by purchasing a slim drive after market. The Panasonic UJ-240 OEM SATA model is the cheapest one available at the moment. This is the bare drive capable of up to 6x BD-RE write and 8x DVD+/-RW DL. It's available from Amazon (paid link) and Ebay. To swap this with your DVD all you need is a screw driver and paper clip. Two rail guides and a bezel need to be transferred from the original. After that its plug and play.
The original Lenovo drive is a Matshita UJ8A0A which is manufactured by Panasonic so we can be confident of this replacement. Windows 7 automatically detects the new drive and finds the right drivers for it. Once you have replaced the bezels and installed it the only other reminder it's a blu-ray is the dark blue tray.