Hi,
In iMovie 10 there is no feature that varies the opacity of a title.
You can do it with a workaround, although it involves a little work. First open a new project. Then open with the Preview app a duplicate of any random photo. Do an Edit/Select All, Edit/Delete. That leaves you with a gray screen that is a transparency. Save that to your desktop. Drag it into the new project that you created. It will look like a black clip. Add your title to the clip. Share out the project. It will appear on your desktop as a movie. Drag the shared out movie into your old project. Place it above the clip in the timeline that you want to add the title to. Select the title clip that you placed above, and click on the Overlay icon (the one that looks like overlapping squares) in the tool bar in the upper right of your screen. In the selection box, choose Cutaway. You will see an opacity slider to the right. Slide it to adjust the opacity of your title.
There is an alternative way. Instead of using an iMovie title you can create your text, using the Preview app, on the transparency screen that you created in the above procedure. Click on Tools/Annotate/Text and just type in your text. Then save and drag into iMovie, using it as a cutaway as before, and adjusting the opacity.
I hope the above is clear.
-- Rich