For those who struggling with installing the "unsigned" APK, to get it linked with Google, here is what worked for me (
kudos to Kaloid for providing some info):
1. Make sure your phone/emulator is rooted (you can download "Root Checker" from the store to check)
2. Install Knighthood, from the Play Store
3. Log in into Knighthood (you should play the tutorial quest without the mod, otherwise it won't trigger the tutorial properly)
4. Install Lucky Patcher (from their website:
Lucky Patcher (download the APK, open it, run it, accept the installation...)
5. Run Lucky Patcher and do the following:
a. Click "Toolbox"
b. Click "Patch to Android"
c. Check "Signature Verification status always True" and "Disable .apk Signature Verification"
d. Click "Apply". Your phone/emulator should restart.
e. Go back into Lucky Patcher > Toolbox > Patch to Android" and check "Disable Signature verification in the package manager".
f. Click "Apply". Your phone/emulator should restart.
6. Run the SIGNED Knighthood APK from BlackMod.
7. Play the modded game.
Here is the process in action, from a fresh Android 7.1 session (Nox emulator). Note that I pre-downloaded the Knighthood APK from the store to save time on the video (it installs while I get started on getting Lucky Patcher)..
Hopefully this helps a few others.