Hi Bert. Ours carry two rings. one white and the other bright green. Not bothered at all. Selecting the rings is a pain though to get the correct size -probably 14mm for your Blackrocks. We use both the clip type, which are quite secure but have no adjustment but don't get wrapped up in their claws, and the coiled strip type (not the one that looks like a spring) which caters for tightness by giving a bit but tends to get caught up in their claws and can slip and damage their feet. The latter can also be pulled off, which is good if they get it wrapped up against something. On balance prefer the latter. Both have flat surfaces than can be written on -which is why they have them, for their names and export numbers.
Size varies bird to bird so you need to buy a variety.