By farming a total of 20,000 Sightless Eyes, you get a permanent Ratstallion mount, so this is a grind many players are currently doing. It does not take all that long to farm 20,000 Sightless Eyes, but be prepared to spend a few hours down in the sewers.
Anyway, in order to farm the Sightless Eyes as quick as possible, you want to join a premade group using the group finder tool. Find a group where the PvP is turned off is the sewers, and kill as many rares as you manage with your group. Once someone turn on PvP again, leave the group and join a new premade farming group to farm more rares. By doing this you will have very little downtime in the farming process, and you should be able to farm the 20,000 Sightless Eyes very fast compared to grinding them out on your own.
If you are doing pet battles, then you could also go into the sewers and do this. Each battle win awards you with 75 Sightless Eyes, so this is a nice alternative strategy. It might be a bit slower, but it is pretty easy once you get the hang of it. Good luck!