Well things like alfalfa and alfalfa pellets are much higher in calcium than kale which you do not feed Mr. Bunny anyway – alfalfa that is.
There is not a whole lot that vets really know about bladder sludge – I have a rabbit with bladder sludge and have had several before with this problem. There are many theories out there concerning bladder sludge and stones including genetics and metabolic bone disease. Basically there is no direct scientific link between calcium in the diet and the amount of calcium excreted in our rabbits urine.
Normally if your rabbit has problems with bladder sludge or stones then your vet will recommend you reduce calcium intake at that time.
My opinion on the kale is that you should always give your rabbits a wide variety of vegetables and if possible rotate them so you would want to include kale in the rotation a few days a week – now I will be honest I do not do this but if I had only 1 or 2 rabbits to feed, I could and would do this but I have many more so I have to buy in bulk pretty much.
Calcium is a necessary mineral for our rabbits as it is for us.
I would also recommend that your rabbit get plenty of fresh water and plenty of exercise.