What could be the cause of external genital itching, with no internal itching or irritation? I've been screened already for stds.

Lots of things. Yeast infections are most common. Allergic reactions to household products are another -- soaps, lotion, laundry detergent, fabric softener. There are several skin disorders like lichen sclerosis and lichen simplex chronicus. These can be diagnosed by biopsy.