I'd use a mixture of mainly sand with just a little cement, as a dry mix. Just sweep it over the flags, and do it several times as it sinks down.
When block paving is laid, there are always some tiny cracks in the joints, and some layers just dust plain sharp sand down into them!
Years ago, there was a spate of new block paving in a London Borough. In one particular area, a depression was noted in a road, often crossed by buses, and on investigation, the blocks were taken up, and a huge void was underneath, several feet deep! The strength of the blocks being laid tightly together had meant that they didn't even sink more than a few millimeters!
I try and keep any moisture away from pointing, as around now, there's enough in the air to make a weak cands mix harden as well as anything!