To say that you want to pass a 3d model from blender into a shader into a GM image is about as incorrect as it gets =P
You need to learn a bit of what's going on under-the-hood in graphics programming to understand how to do this stuff.
Firstly, you have your hardware. Your CPU and GPU. The CPU controls all of the logic in your game. It can also be used to initially load a file into Memory (RAM). When you want to draw anything (everything), the CPU tells the GPU that it would like to draw something. The GPU waits for the data and then
draws it to a buffer. This buffer is then copied to the screen and you can visibly see it.
In the past, there weren't things like shaders, the rendering library did everything for you (DirectX/OpenGL). However, this is no longer the case. What you now have are shader programs. These are bits of code that you write, that are then compiled at run-time and sent off to the GPU. When you draw a bit of geometry (could be 3d or 2d they are both the same), it will run through your shader; Once
for all vertices, once
for all rasterized pixels.
Then when this stage is done all pixels that are outputted from the fragment shader will appear on the screen buffer (or whichever buffer is currently bound).
When you want to get a 3d model to draw. You need to first use Game Maker to define a 3d model. There are different ways you can do this. In old versions of Game Maker you are limited to using
d3d_model_create(), and then populating it with vertices. In newer versions you have the
vertex_buffers, which are much more powerful.
After a 3d model is defined, it is sent to the GPU and kept there for the duration of your program. When you draw it, the CPU just tells the GPU that you want to draw that model that is already loaded into the GPU's VRAM.
If you want to load an external model, you need to look into the actual model format. I believe someone wrote a blender-to-gamemaker importer (though I cannot find it).
There is also a Blender-to-gamemaker model exporter.
TheSnidr also released a nice model format that works with Game Maker nicely.
Game Maker doesn't do any magic. Its renderer works like a renderer in any other language (except it is much more limited feature-wise).