If you have problems getting a Montserrat-Black to work:

In this lesson, I'll be introducing you to Google fonts. Some students have had issues getting the bolder Montserrat black to show up. In order to get the different font weights of the Google font, you need to include it when you select it in Google fonts.

When you are about to embed the font into your web page, switch to the Customise tab and select the font weights you need. So if you need thin and black, be sure to check those boxes. Then in the CSS, instead of specifying Montserrat-Black, try setting the font-weight to 900.