Can the doctor diagnose candidiasis with a colonoscopy, or blood tests?

No: Upper endoscopy will disclose candida of the mouth and esophagus. Candida almost never infects the colon. Of course you can have genital yeast infections which are different.

Clinical part needed: The diagnosis of candidiasis is not made without the clinical context- therefore, you can't diagnose it by colonoscopy. However, if Candida is growing from a blood culture, it's never as contaminant and must be treated, and the source controlled (eg. line-related vs GI source vs prosthetic device infection). In many cases, a patient may have other major illnesses affecting the immune system.