Why does my map app go dark and leave me without a clue after dark?

Do you have it providing you with directions? If so, you can go to Settings>Maps>Navigation & Guidance and make sure that Navigation Prompts Wake Device is turned on. If you are not connected to power it can go to sleep, but it will make sure you know about upcoming turns, etc. If you need something else, you can tap on the screen. Unfortunately, I alway have mine plugged in and it stays on the screen unless there is a long period between turns and it will go to sleep for a while.

That is not normal behavior. If you are coming up to a turn, it usually lights up for me about a mile ahead. I would take that up with Apple Support. Use the Contact Support link at the top of the page here for help.

When you say going dark, do you mean that the screen is completely dark, or is it that the map is in a dark mode?