Of all of the different mitzvot that the sages instituted, there are seven in particular that get singled out. They are known as the seven rabbinic mitzvot, and are so delineated from the others because those seven necessitate the recitation of a blessing (source).
I have been wondering about the obligation to eat maror on erev Pesach. The gemara (Pesachim 120a) provides two opinions as regards the status of this mitzva, but both opinions (Rava and Rav Acha bar Yaakov) hold that the obligation to eat maror is rabbinic only. And, in fact, we do recite a blessing when we get to that part of the seder (בא"י אמ"ה אקב"ו על אכילת מרור)! So doesn't this mean that there are eight rabbinic mitzvot, not seven?
In short: If the obligation to eat maror on Pesach it is not a rabbinic mitzva, there shouldn't be a brakha; if it is a rabbinic mitzva, why do we say that there are only seven rabbinic mitzvot?