Problems with lower thirds
Assuming that your video file does not have an alpha channel (an extra channel of information in the video file that defines which bits of the video should be transparent):

Place the text .mov on a layer in your sequence
Select the clip
Go to the Effect Controls for that clip
Under the &quot;opacity&quot; heading, set the Blend Mode to Screen.

Anything black should now be transparent.

If you think your file does have an alpha channel, check that the alpha is set correctly in Premiere:

Right click on your clip in your project
Choose Modify --&gt; Interpret Footage
Make sure the Alpha settings are set as shown: