When I played with blackhole's "bh install" command in osx, it stops at "Downloading packages". After I look into the source code, it seems the call to "with-output-to-process" in "untar" function causes this.
Below are code I wrote to reproduce the problem
(with-output-to-process (list path: "sed" arguments: '("-e" "s/n/N/g")) (lambda () (display "nothing")))