First use DDU to uninstall all traces of the Nvidia GPU driver which might cause conflict with the AMD Driver then install the current AMD driver using this basic method: (NOTE: You can use this method for both Nvidia and AMD drivers)
Uninstall the current AMD Driver using DDU as per this method:
It could be due to a corrupted AMD Driver installation.
Download free program DDU (Display Driver Uninstaller). Run it in Safe mode with the internet disconnected.
You can also run it in Windows Desktop but then you would need to Reboot the computer for DDU to finish erasing all of the AMD Driver traces from your computer.
Once it finishes uninstalling the current AMD Driver in Safe mode and boots back to Windows Desktop, still with the Internet disconnected, delete the AMD Installation folder C:\AMD if it was created before.
Now install the full AMD Drive package you downloaded manually from AMD Download page. Not the express package. Run the package and if it installs correctly again delete C:\AMD folder and reconnect the internet.
This should prevent any conflict when installing the new version from the previous version.
By the way, C:\AMD is always created whenever you run the AMD Driver package
Here is the latest AMD Driver for your HD7870: https://www.amd.com/en/support/graphics/amd-radeon-hd/amd-radeon-hd-7000-series/amd-radeon-hd-7870-g...
Make sure your Motherboard has a PCIex16 -8x Slot.
If you are still getting black screens then try installing a Previous AMD Driver and see if it makes any difference from here: https://www.amd.com/en/support/previous-drivers/graphics/amd-radeon-hd/amd-radeon-hd-7000-series/amd...
Try using a different GPU Video Output and see if you get video or not.
Also is your GPU card recognized in Device Manager under Display?
What does GPU-Z show after installing the AMD driver if possible?
When it goes black when booting up do you get any Error messages such a "No Signal" ?
If you still are getting a black screen try it in another computer. If the same thing happens then you might consider that you got a defective used GPU card.