Noggy is correct, Fazer Bluespot rears are rare and £££ but fit onto the standard rear mounting plate with a bit of filing, the early R1 bluespot rear DOES NOT & no-one knows if the XJR1300 bluespot rear fits or not.
The alternative involves swapping over to the TRX850 rear mounting plate. You also need a replacement torque arm and preferably the donor bike's brake hose as they are usually a bit longer. But strictly speaking, you don't have to have a matching TRX850 rear caliper. Quite a few mid-late 1990's Yams with underslung calipers also fit, such as YZF750 and YZF1000 Thunderace. For example, I have a Thunderace rear caliper shown below fitted to my own (grotty but £15 complete with decent pads).
Don't be put off by the alternatives to bluespots, all the above calipers (FZS1000, TRX & YZFs) share 43mm diameter pistons compared to the original FZS600's 38mm pistons so offer superior feel & stopping power.