From the picture it looks like you are not dealing with a blight, but rather an insect problem. The trees look like they are now hosting a poplar borer. The brown sap/stain you see is the trees defense mechanism against the borer. Poplar borers are quite common in Aspen, but also can be difficult to control because they are located in the heartwood of the tree. Utah State University has a good bulletin on Poplar Borers including control methods. Here is the link. https://extension.usu.edu/files/publications/factsheet/poplar-borers01.pdf