I'd like to drive from Heiligenhafen to Saarbrücken (Germany) on the A1 (Bundesautobahn 1), but there is an interruption between Blankenheim/Tondorf and Kelberg.
Is there an alternative way between these towns?
You are talking about Germany, of course there are alternatives.
For so long a journey you have the choice out of several long distance motorways, Google finds both the A1 you plan to use and the A7, which they recon is a few minutes faster. And it is not coming near Blankenheim/Tondorf or Kelberg.
But does taking the A7 make you miss all works on the motorways? Not for sure, as both alternatives use the same stretch of road till just south of Hamburg, with a warning for road works on it. And the A7 has warning markers south of Frankfurt am Main.
Your best bet is to use a road map online with roadwork markers and look for local alternatives. There are plenty of smaller roads near the road works markers, for all locations, but which will be acceptable for you depends on your personal wishes, dislikes and the road conditions at the time you need to travel.
I used Google maps for a quick check and it allows you to alter the route by just dragging the marker line to a road you would like to use, which can be a tiny local road or an other main road. I know other online maps also have ways to change routes as easily.
In your satnav/GPS you can often just enter an extra destination, which should be on the alternative road, and when you are almost there you take it out again.
If you can read German, search online for a local site with road works, not just on the main motorway but also on the smaller roads.
Check on how much delay you can expect at each of the road works, as @Jan comments, many of the road works are not worth diverting for. As they do not slow you down much.
A tool that would work much wider than just Germany is Waze, as indicated by @Zach Lipton, which is an app that allows people to share traffic information.