Does one need to say a berachah on the aroma of herbal teas?
Yes, if one smells a herbal tea with the intention of enjoying its scent, one should recite a berachah. If one takes the tea to drink it, and only smells the scent by the way, one would not make a blessing. Note that the scent of ordinary teas is not strong enough to warrant a blessing upon “enjoying the scent,” and only if the tea is truly fragrant would one recite a blessing.
The blessing that one makes would be the appropriate blessing for the herb, and hence would usually be borei isbei besamim.