Hi Musickey,
I do not know what dark paper you are using, but I use Ironall Dark. Temperture and times can vary from paper to paper. Even with the same paper, it can vary from user to user, depending on how hot your press runs, and what paper you use.
For dark paper in general, I would try settings of 375*F which is closer to 190*Celcius. At that temp, I only press for 6 seconds. Some people go to 8 or 10 seconds. Others use a temp of 350*F which is closer to 176*Celcium, but they press around 13-16 seconds.
The 180*Celcius you are using is around 357*F, which is still okay, but you need to back down the time you let the shirt dwell under the press.
I think you are pressing for too long. When dark paper is too hot for too long, it will have troubles, among them it loses it's opaqueness and even starts to become transparent.
You can leave the same setting but try to cut the press time to around 10 - 15 seconds. Kind of experiment.
Here is another post on pressing. Part of it covers dark papers. I don't know if it will help further, but it might. Good luck to you, hope it works out.
transfer paper tips for lasting image - Page 5 - T-Shirt Forums