Not necessarily -- "combustion" generally implies a self-sustained oxidization reaction, whereas "blackening" merely indicates that a number of the more complex molecules have been broken down -- "reduced" to carbon. For example, it is possible to blacken nearly any organic materials in the complete absence of oxygen -- a clear distinction/demonstration that combustion is definitely not required for blacking -- ie, the terms/events are identifiably distinct. You will know for sure when combustion occurs because it always results in ash -- generally a gray powdery residue.
However, blackening is not really that desirable either, for it indicates that at least potentially some of the medicinal ingredients have been overheated, reducing them to less useful compounds -- ie that less than optimal efficiency has been obtained. Usually, it is recommended that frequent stirring between draws be used to prevent blackening, particularly as it improves the overall taste.