Black to Translucent Gradient

All you need to know, is that InDesign isnt like Illustrator regarding fade from something to transparent – you have to select a color and you cant control the opacity.

The way John explains is a global approach/workaround to use in various gfx-programs.
Like said above, your tool is a 'effect' (Windows » Effects). Here you can control some options with values which you cant control just with the gradient tool. To add a effect to lets say just the box but not the text inside, you can control it by applying the effect individually.

Doubleclick on the part you want to assign.

Another option that may produce a desirable result, depending on the underlying image, would be to set a black-to-white gradient and set its blend mode to Multiply.

You use the transparency as gradient feather on the frane you want to use, you can also drag with the gradient feather tool: the chessboard icon in your tools. There is no option to add transparency inside a (gradient) colour itself.