'Blooming' is a mild expletive that is commonly used in the United Kingdom and some other Commonwealth countries. However, as in the case of the two examples that you provided as part of your definition, 'blooming', when used as a mild expletive, is not followed by a comma. Its use is widely accepted in informal conversation, but should be used with discretion in more polite society and should rarely, if ever, be used in formal writing. The definition that you provided used this sense of the word.
'Blooming' in the sense used by Dumbledore, means 'in good health'. It is commonly used to describe women when the are pregnant. In this case 'Blooming,...' is an abbreviation for 'I am blooming,...', so it is rightly followed by a comma. For me, this is an unusual adjective to use to describe a male, although I would not say that using it to do so is incorrect. Most men would normally have responded by saying something like, 'I am well,...', or perhaps just, 'Fine,...'.