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",
"black": "#2bff00",
"blue": "#17b2ff",
"brightBlack": "#749B36",
"brightBlue": "#27B2F6",
"brightCyan": "#13A8C0",
"brightGreen": "#89AF50",
"brightPurple": "#F2A20A",
"brightRed": "#f7982d",
"brightWhite": "#ffff00",
"brightYellow": "#09e24a",
"cyan": "#3C96A6",
"foreground": "#fcf9f9",
"green": "#6AAE08",
"purple": "#991070",
"red": "#8D0C0C",
"white": "#6E386E",
"yellow": "#991070"
}