We have a hairline crack (< 1/32") about 15 feet long in our basmenet wall. Crack is about 3 to 4 rows of block from the top of the foundation, or about 3 feet below grade, right at frost line. Crack is staggered. Wall is not bowed as best I can tell with a 4 foot level. What's a good repair for this? Just fill it with grout?

It should properly be repaired inside the basement and outside the basement. If it means digging outside the basement wall down to the area on the outside, then the fix on the inside would work much better and keep the block from totally crumbling. You need to also address any water retention problems you might have with the foundation as well to prevent any further problems with the block wall.

Is the crack in the mortar line only or does it go through the concrete blocks?

It's only in the mortar, hardly noticeable, we're getting ready to sell our house and a home inspector found it. I guess I can upload a picture of it.

If its only the mortar line its likely just settling and tucking with grout should be fine. I would get a masons 8' level or a very ridgid stick of angle iron and periodically check the wall top to bottom to insure it is not bowing. If it bows call us back.