I've had blood work done very rarely. I have 2 old ones, 30 and 32.The old one colicked about 3 weeks ago, and recovered; the vet said if he goes off his feed we need to do blood work to see if his liver or other organs are slowing down. (That might have caused the colic in the first place.) So, if he loses his appetite again I will definitely call the vet for blood work. I expect it will cost more for the vet trip ($60) than the blood work. But, it will tell us whether we are dealing with something simple or life-changing.
Sometimes they want to do bloodwork before they anesthetize or sedate the older ones, just to make sure everything is okay before they start. I've used the same vet for years, so he knows my horses and knows I keep an eye on them, so he's not doing routine blood work on them yet.