Hi MoniLink, for 5 days Holiday you don't need a Visa according to my information.
Basically you are allowed to stay in any EU Country for up to 3 months within a period of 6 months (as long as you don't work there) given that you are a recognized refugee with a blue passport.
Event though Bulgaria is not part of Schengen it does not require a Visa for German residence titles such as jours. There will be a passport control though but your blue passport should be sufficient.
I would recommend you to double check with the embassy or General consulate of Bulgaria here in Germany to make sure they have not introduced any restrictive special rules for refugees in the meantime. And maybe also call the Airline just in case.