Belgium rejected my visa due to "Justification for the purpose and conditions of the intended stay was not provided". I have since obtained a visa from Germany. Can I exit the Schengen area via Belgium (Brussels) or do I have to change my flight ticket and exit from another Schengen country?
What is contained in the (first) field Valid for of the visa?
If it contains Schengener Staaten (having been issued by Germany) and nothing else, then the visa is valid for all Schengen countries (including Belgium).
If the field contains anything else, please add that information to your question (or an image the visa with your personal information blotted out).