Yes, you can do this, but what you’re looking to use is a reversed luminance key, or a change in the compositing mode for the text track and the clouds track under it.
Here’s how you do it with a blend mode or compositing mode change:
Put the text track of white text / black background, on the top layer. Fat, wide fonts works best for this.
Now put the clouds track, either motion video or a still, on the video layer just under the text layer. Set your play head in the middle of both clips.
Select the top, text layer with a double-click, then go to the drop-down at the top of the computer screen labeled: “Modify” and scroll down to “Composite Mode” and “alpha type” Select Alpha type of “WHITE” or “STRAIGHT” and then, if needed, scroll down to “reverse alpha” and click that.
What that does is blend in the clouds ONLY in the areas where the brightness of the white text layer is high. If you had a complex shaped logo that had solid white in it, instead of text, you could selectively fill that area with clouds, using this method. Or flames. Or bubbling water, or a wood grain texture or metallic looking texture… anything, really.
Render this out (EXPORT) as it’s own, self-contained clip, called CLOUDTEXT, then bring that clip back into the editing bin with an IMPORT command, and now, you should be able to luma key it over any other video.
To do that, take the rendered, self-contained clip of cloud-words over black that you just made, called CLOUDTEXT, and stack it over the footage you want, double-click it to select it, go to EFFECTS> VIDEO FILTERS>KEYERS> LUMINANCE KEY, and in the FILTER control panel for that clip, set KEY MODE to “KEY OUT DARKER”. You may have to adjust the THRESHOLD and TOLERANCE sliders.
There are other ways to do this as well, but this is how I usually handle it.
When you have the time, stack any two clips, one over the other, select the top track, and play with ALL the composite mode “flavors” and see what they do: they can be powerful, yet fast and easy effects to use.