The only cancer that might accidentally be discovered via blood test looking for something else is leukemia. That's how my husband's leukemia was discovered. There are multiple kinds of leukemia and not all would have been noted, but he had almost no white blood cells when they were looking for a white blood cell count because he wasn't recovering from an illness like he should have been. If your test revealed or confirmed an infection, then you don't have that kind of leukemia.
Other cancers would not be revealed by a simple blood test. Some cancers leave tracers of one kind or another in the blood, but a simple test is not looking for them.
An abnormal growth in the area they're doing the ultrasound on might be visible to the technician, but s/he isn't a doctor and will only note it on the chart, getting images from as many angles as possible, but not discussing it with you because they're not allowed to give diagnoses.