blessing - Who Commanded Us, and For What?

Whenever a Jew performs an activity that is mandated by the Torah, s/he recites the blessing that acknowledges God, "אשר קדשנו במצוותיו" (who sanctified us with his commandments), and who commanded us to... [insert name of activity here].

This is all well and good when the activity can either be found explicitly in the Torah (eg: blowing a shofar) or can be inferred from the Torah (eg: waving the four species), but what about those things for which there is nothing in the Torah at all?

By way of an example, washing one's hands before eating bread, or lighting candles on erev Shabbat. Both of these are rabbinic enactments; they have no basis whatsoever within the actual written Torah. I know that it is necessary to make the above blessing when performing them ("Blessed are you, O God, etc, who commanded us concerning the washing of the hands, etc") - my question is why? It was not God, strictly speaking, who "commanded" us in this regard, but the rabbonim.

Furthermore, to the best of my knowledge, we do not make this blessing when writing a prozbul, yet that is no less rabbinic in nature.

Answer

1) See Shabbat 23a, which discusses Menorah on Chanukah:

מברך אשר קדשנו במצותיו וצונו להדליק נר של חנוכה והיכן צונו רב אויא אמר מלא תסור רב נחמיה אמר שאל אביך ויגדך זקניך ויאמרו לך

Soncino translation:

What benediction is uttered? — This: Who sanctified us by His commandments and commanded us to kindle the light of Hanukkah. And where did He command us?— R. Awia said: [It follows] from, thou shalt not turn aside [from the sentence Which they shall shew thee]. R. Nehemiah quoted: Ask thy father, and he will shew thee; Thine elders, and they will tell thee.

2) I don't think the prozbul is technically a mitzvah- it's a תקנה- a rabbinic decree designed to improve the general welfare of society. (Also note that according to the Rambam (שמיטה ויובל ט:טו), the prozbul is only effective when the Sabbatical year is only observed on the rabbinic level. When the Sabbatical year returns to a biblical obligation, the prozbul enactment will be void.)