What does it mean mildly positive to ischemia?

Arterial blockage. Ischemia in areas of the heart, as you have related, infers that the arteries that feed these parts of the heart are narrowed such that blood flow through them reduces with exertion. It means these areas of the heart are vulnerable and further testing is needed and intervention may be necessary.
Blocked artery maybe. I'm assuming that this refers to the result of a stress test in answering this question. Ischemia means that part of the heart is not getting sufficient blood flow. The mildly positive part of the result suggests that the interpreting doctor thought that either the area of the heart involved is small, or the degree of abnormality seen on the test was small. This could be due to a narrowed artery.