example

my block

I would like to add black block after red one with text, similiar like on the example (first screenshot)

I was trying to do it like that, but it doesn’t work. I’m new to CSS… :/

h2 {
    width: fit-content;
    background-color:#FF0000;
}
h2:after {
    display: block;
    width: 50px;
    background-color:#000000;
}

How can I achive something like this?