This is a great question!
Vincent is exactly right in that a hover interaction wouldn't work on mobile, simply because there is no way to hover on a touch device.
This is what I recommend doing:
Keep it so the overlay is always visible. Yes it will cover the image, but I'm most cases the text is more important, and should be shown over the image. This is also a reason to instead of doing the initial appearance in the Interaction panel, to simply change the way the item appears on the Style tab (in this case, instead of adding an initial appearance of 0% opacity within the interaction, simply change the opacity on the element itself in the style panel). The reason for this is that currently Interactions doesn't let you change the initial appearance for different devices, so if the initial appearance is set as 0% in the Interaction, it would be that way on all devices - and since mobile doesn't have hover, there isn't a way for the viewer to see the content.
Im going to update the blog post to talk about this as well!
Thanks for the feedback!