In the Black Market - Dock Services - Weapons section the actionDesc for 'Buy Upgrade' is "We do not have any weapons for sale". This shows even with an actinide cannon in the station inventory. A more accurate actionDesc would be "We do not install weapons". Seen in both BM station types.

I didn't test it but this actionDesc should probably cover armor and all devices except cargo holds as well. From memory the BM stations only install holds.