What I've used for basic Blockland modeling:
Pencil drawn concept art (The side of the weapon).
A method to digitally attain that image to use as a background in the blender 3d view.
I've used an iphone camera and dropbox and even directly getting the image from the iphone.
I suggest that you study the standard "blocko" look before modeling.
Basically this, look at how the blockhead's arm doesn't extend too far from his body. This makes the gun look stumpy if you wish the gun to be to scale with a blockhead. Look at Bushido's model style. Notice what level of detail his weapons have. Generaly, try to make your model as "semi-flat" as possible. For example, if you want a round barrel for a gun, don't make some 32 edged circle. That is too much. Instead, try a hexagonal or octagonal cylindar for the barrel. It gives the barrel a round look but still looks "blocky". Also, make sure your gun isn't too long either. Remember, "Stumpy, low poly, short, and semi-squarish/squarish shape". I've noticed that people tend to be too realisticaly scaled in their weapon model and it doesn't look good. The gun needs to look like it fits a blockhead.