Just add it to themes section of your settings.json file. Read more here-> https://docs.microsoft.com/en-us/windows/terminal/customize-settings/profile-settings/#color-settings. Replace 'colorscheme' parameter name to Blackfrost.
To make your terminal completely awesome, follow https://docs.microsoft.com/en-us/windows/terminal/custom-terminal-gallery/frosted-glass-theme tutorial and replace the frost theme by Blackfrost
{
"name": "Blackfrost",
"background": "#000000",