Hmm... Did you verify that you typed this command in the exact directory the blackarch package was downloaded to or moved to?
A common solution to the "target not found" error, is to find the package/file, right click on your mouse and select "open terminal here".
This will cause the terminal to open and be in the directory where the package/file is.
For example, the below says [sux0r@ ~]$, but if the file is in [sux0r@~]/Desktops$, then it will not find the file.
tried both got this:
[sux0r@ ~]$ sudo pacman -S blackarch-config-xfce
error: target not found: blackarch-config-xfce
[sux0r@ ~]$ sudo pacman -S blackarch-menu
error: target not found: blackarch-menu