If they are insects or insect droppings, you can just hose them off and forget about it. The leaf closer to the bottom of the photo has a few whiteflies. The black insects might be leafminer flies. In any case, hosing them off is the first thing to try.

Here is a link that might be useful: Citrus pests

They are black citrus aphids; normally you can just wash them off. The bigger issue is when you have aphids on lemons, it means you have ants. If you control the ants, you won't have the aphid problem.