Apple increased security in macOS Mojave (10.14) and Catalina (10.15) requiring the user to give third party applications "Full Disk Access" in order to access data such as Time Machine backups, Messages, Mail (email), Safari, Home and certain administrative settings. This impacts BlackLight users processing this kind of data from an attached disk, including an attached Mac in Target Disk Mode. Follow the steps below to give BlackLight "Full Disk Access" on your Mojave or Catalina machine.
1- Quit BlackLight
2- Go to the [Apple Menu] and open [System Preferences]
3- Select on [Security & Privacy] and then click on the Privacy tab
4- Click on the padlock in the lower left corner and enter admin credentials
5- Select on 'Full Disk Access' in the left margin
6- Click on the + symbol under the right pane
7- Navigate to the version of BlackLight you are using, click Open. BlackLight will appear in the right pane.
Now you can launch BlackLight and have full disk access to attached disks.