Which is correct in this situation?

You were right to choose “I have to wait” because at a German roundabout marked with the roundabout sign (traffic sign 1) the basic rule is: traffic already circulating in the roundabout has priority, and anyone entering must give way.

So if you are approaching the entry while the blue car is already in the roundabout (or would reach the conflict point first while circulating), you must wait at the give-way line and only enter when you can do so without forcing them to brake or change course. This is why “the blue car has to wait” is wrong in this situation: vehicles inside the roundabout do not yield to vehicles entering.

You also don’t need to “reach an understanding” here, because the priority is clearly regulated by the roundabout rule. “Reaching an understanding” is only for unclear, unregulated situations (for example very narrow roads with no signs where two drivers cannot pass).

Rule of thumb for similar questions: At signed roundabouts in Germany, entering traffic yields; circulating traffic goes first. Also, you do not indicate when entering, and you indicate right only when you want to exit.