You need to read the licenses of the software you're considering.
Some truly free options:
Surprised not to see Art of Illusion mentioned:
It’s described somewhere as the default design software for the RepRap project.
List of other options here: http://www.shapeoko.com/wiki/index.php/CAD
While it's mostly oriented towards CNC (subtractive milling), there's a list of free/opensource options here:
http://www.shapeoko.com/wiki/index.php/CAD#3D
In particular, you may want to try Art of Illusion: http://www.artofillusion.org/
I don't use Blender, so I can't really help you with it. The only experience I have with creating meshes is using a program called Art of Illusion and even then I have no idea if you can import what you created with that into Roblox ¯\_(ツ)_/¯
I'm no good at art either. I eventually found that I got better results by making models in Art Of Illusion and putting the rendered results in my game, and animating them with code (by deforming the meshes). There are sometimes code-based solutions to these problems. Alternatively, look for a style that doesn't require much drawing skill. That's largely why pixel art is so popular, but it's not the only possibility. My game Balloon Platform Defense only gave me difficulty when doing the human characters (which is when I used the solution above) - everything else was made of basic shapes. And earlier I was playtesting a game called Supercoil - I doubt the author of that worried much about his drawing skills either. Try to find a game theme where most stuff is made of primitive shapes, or where little art is required.