Is it possible to trace blueprints using curves in blender i am attempting to model a car and wolud like to use curves to define the proportions of the vehicle thanks in advance.
I don’t think Blender will allow you to do that - unless precision is not an issue. (In which case, you can use it as a background image to trace over.)
I’d suggest you do it in Inkscape and export.
actually you can, using the various curve tools to build your lines is fairly straightforward - add curve, then edit mode, then move the initial curve into palce, then select the last point, and ‘e’ out the next point(s)… you’ll need multiple curves - as they can’t branch… then you can convert them to poly’s… its almost easier to just use poly edges though,
What about Nurbs?
And, actually, I was basically replying to his question about snapping to the image - which, I think, he can do in Inkscape. But you’re probably right - since his aim is to get the proportions right - not exactness. 
you can’t really snap with a nurbs curve or surface, as the control points are the things that will snap, not the point on the surface… you can snap with blender, you dont need inkscape for this - the little horse shoe magnet in the 3D window turns it on, then hold control and move… you can snap in any axis in isolation, to align things, or, turn on automerge editing in the mesh menu, and you can snap to points and have them ,…err… automerge… (i have an autocad background and from that, any graphical program without snapping is useless to me now… :D)