I presume those bits of wood sticking through are your kitchen floor joists?
You need to make sure you don't potentially bridge any damp proof course so my actions would be as follows;
Fix a batten with the top edge level with the top edge of the lowest blocks, (i.e. the top edge of the dark ones). The batten needs to be such a thickness that, when you hold a vertical batten/spirit level across the gap, they are flush.
Mix up a mortar & cement mix, (4 mortar : 1 cement should be adequate), and fill up the gap with the mix.
Using a straight edge board/batten slide it up and down whilst working across from left to right, (or right to left which ever suits you), levelling off the filler. Allow to dry for a few days then remove the support batten fitted to the lower blocks.
If you are not putting anything against that wall you could always fit a bit of skirting to hide the lower blocks or just leave them and paint over when you re-paint the rest of the walls.
If that dark strip across the floor is damp proof membrane, then you can take the filler down to floor level. Just set the support batten on the floor, do the above and then remove the batten and fill in the slot left by the batten.