If nothing comes out of blood work up for iritis, then is it assumed that the cause is unknown?had 3 flare ups in 9months. bloodwork neg. next steps?

Depends: Depends on the blood work, if it is a relatively complete work up then you would be classified as idiopathic (unknown). However, approximately 50% of iritis cases are idiopathic. Treatment depends on exam findings so it would depend on what your eye specialist feels is necessary to control your inflammation in the eye.

See a rheumatologist: Testing done under the supervision of a rheumatologist and ophthalmologist may help to determine the cause. And maybe not. It may take a long time to determine the cause of iritis, but there is a cause.