I have to disagree with the two of you on that...or at least ask why you consider it unprofessional?
A professional uses the right tools for the job. Anything else is a kludge. It may get the job done, but it's not exactly the most professional (or elegant) way to do it.
Blacking out or otherwise covering the windows is certainly better than leaving them open, but it's tacky and unprofessional. This is, of course, purely my opinion. But, it's not at all unlike loading the dash with papers, maps, pens, pencils, books, magazines and all manner of assorted crap. It looks tacky, unrefined, unorganized, unprofessional.
Many cargo vans have rear windows. But they aren't really cargo vans. They're work vans that are used by plumbers, electricians, mechanics, or in the case of my first van, a fire extinguisher technician who repaired and renewed fire extinguishers. I blacked out the rear windows with glued-on Reflectix bubble pack. It worked well enough, but I had no illusions of just how professional it looked.
Again, this is purely my opinion, and it's worth exactly what you paid for it. But I cannot believe that I'm the only one who shares that opinion, so it goes back to my original statement that it comes down to under which level of professionalism you wish to project and conduct yourself. If you are comfortable projecting "professional enough", then that's what you should do.
You know those vans you see every now and then with the big, honkin' gaudy looking padlock on the rear doors? Unprofessional. Tacky. Inelegant. Use the right tool for the job, and the right tool for that job isn't a cargo van with a tacky looking aftermarket lock, it's a box truck with a proper locking system or a professionally modified cargo van with the proper body and doors.
But that's just me. But there's no getting around the fact that your level of professionalism is reflected in everything you do and in how you present yourself and your tools.