In BA we use an enzyme type "sink un-blocker" but is not much use in a totally blocked sink.
I must say, I was pleased to see a label in the rear galley in one B767 saying to pour hot water down the sink.
As for the design of sink drains, they could be better! The B767 for example has sharp angles in the drain line, which doesn't help the flow of liquids. Aircraft are still a long way from being maintenance friendly
Though I am being paid a vast wage to work these unfriendly aircraft. LOL