By the time cancer is getting close to/effecting blood, you're pretty far along for some cancers.
By sequencing the abnormal DNA that a tumor releases into a person’s
bloodstream, researchers are now one step closer to a universal cancer
test. Although the technique is now only sensitive enough to detect
advanced cancers, that may be a matter of money: As sequencing costs
decrease, the developers of the method say, the test could eventually
pick up early tumors as well.
I didn't say just ones that effect the blood. Getting to a blood supply is pretty bad too, makes it so the tumor can grow more. Many non-blood cancers can spread via the blood, which is also very bad.