Try this

Place a black rectangle over slide 1 and give it an exit animation of fade /
very slow

Do the same on last slide but make it an entrance fade

What John said.
It is also possible to fade in the first slide without using black
rectangle. Click Slide Show > Slide Transition. Set the transition as Fade
Through Black.

As for fading out of last slide, you can add an additional slide after the
last one, and set it to black background. Next, set the transition to Fade
Through Black and Advance Automatically after 00:00. This way, you do not
have to add a black rectangle which will prevent you from viewing and editing
objects underneath.