Learn Dutch: Een verfrissende geur is goed voor de bloeddruk. - A refreshing scent is good for the blood pressure.

Because geur is a de-word (de geur). Adjectives before singular de-words always take an -e ending, regardless of whether you use de or een. The base word here is the present participle verfrissend (refreshing), which acts just like a regular adjective.

Could I use the word stank instead of geur here?

Only if you wanted to say a bad smell is good for blood pressure! Geur is neutral or positive (scent/smell), whereas stank specifically means a stench or foul odor. Since a refreshing smell is positive, geur is the right choice.

Why is bloeddruk written as one word?

In Dutch, compound nouns are glued together into a single word without spaces. It combines bloed (blood) and druk (pressure). The article of a compound noun always comes from its final part, which is why it is a de-word (from de druk).

How could I express this same idea using the maken dat (causes to) structure from this lesson?

You could say something like Een verfrissende geur maakt dat de bloeddruk verbetert (A refreshing scent makes the blood pressure improve). Remember that dat introduces a subordinate clause, which pushes the conjugated verb (verbetert) to the very end of the sentence.