Perhaps I should have been more clear before but I'm not getting married in a barn. It's a historic Virginia home with a horse farm on the grounds. My concerns with black tie are mainly due to the heat - the average daytime high that time of year is around 80, though it should cool off when the sun goes down.
I assumed it was as you describe--my wife and I also got married in the D.C. area and looked at plenty such venues. I'm sure it's quite elegant. However, the appeal of such a place is precisely that it is also in the country. Hence, black tie is inherently at odds with it. A dinner suit is really more of an in-town kind of get-up.
Now, if you have everyone wear black tie, that's a different ball game. Then you are creating an environment suitable for dinner suits, country or not. If it's just the groomsmen, it looks like you just don't know what you're doing and trying very hard to look like you do.