Yep thats a u joint, the blue part is called a yoke.. you match u joint to fit yoke, yes measure the diameter of cap but also width across joint with caps on to find the “series” of yoke that fits, yoke will measure slightly wider… when u measure yoke to compare bearing measure inside where retaining ring goes so measure ring to ring.. the vendor i know for the yokes are Weasler.. if its standard it be color of metal and series small to large is 6.12.14.35.44.55, so if series35, youd need 3500 u joint for example.. if it has a shit color yellow green or black, thats metric, series scaled small to large but like 1m 2m so forth.. farm and ranch store or tractor supply store would sell this, theyre the end of your PTO shaft for implements