The other three mobs have the same colour as they have in the level selection screen.

This colour also relates to their weakness.

The Blurs have a weird green/yellow/olive colour on the minimap.

I think that this is a bit weird as with the others you can see with a glance what their weak against and you can easily distinguisch them on the map.

For me the Blops and the Blurs are to close in colour on the minimap and it also doesn't make sense that they are the only one with completly different colour on the map compared to the one in the level select.

I agree, and it seems the icons are also placeholders that were never removed. Everything else on the map looks polished and properly illustrated. I wonder how the blobs and bugs look to people with red-green colour blindness? The assets exist; I'm not sure why they haven't been used for the HUD/minimap.