I recommend the Pioneer 6x Silver External Slim Portable USB 3.0 BD/DVD/CD Burner Model BDR-XD05S, currently $85 + $2 shipping at newegg.com.
It is compatible with USB 3 Super Speed mode, which virtually guarantees the burner will work at full, rated speed. It does, however, require a second USB port if the drive is connected via USB 2.
If price is a issue,...
Yes, OWC does in fact make such Blu-ray drives for Macs.
If you just want read (not write) support for Blu-ray disks, there’s the OWC Slim 6X Portable USB 2.0 Blu-ray Reader.
If you want both read and write support for Blu-ray disks, there’s the OWC Slim 6X Portable USB 2.0 Blu-ray Burner.
Keep in mind they use a USB 'Y'-type cable, so you’ll need two ...