I've just started picking up blender for some projects at work, and am hitting a stumbling block. I'm building a fairly mechanical object and as such have been creating it in layers, using extrusions from paths. All this seems to have worked fine however when I'm now trying to use a boolean to "cut" holes into them, it leaves open holes into the object. Attached are some images to try and illustrate what I mean.

My overall Object/project
Here you can see me selecting my boolean modifier
Here we have the result. Weirdly, it seems to add it
So I go ahead and delete the extra faces, and I've got holes aplenty
You can see on the front of the first image a boolean that I used to cut away a cruve from a block that worked fine, so I'm stumped.
I wonder if it has something to do with my objects being blue rather than grey, but I don't know if this even means anything.