There is no fully "trusted" source of the rate. We do have rates from the leading stock exchanges, we do have average or weighted aggregates of those rates. But still that sources are "points of weakness" in the system. We can't trust them that way like we trust blockchain information.
No offense, but you're absolutely incorrect. Market prices are the only "real" prices. Please understand the concept of "price discovery".
https://en.wikipedia.org/wiki/Price_discoveryEven Fiat like USD, has no "official" price in relation to other currencies. Various exchanges and
banks trade currencies and rates are always floating. Rates tend to converge on a single price at any given time
due to arbitrage. This is true whether its dollars or Bitcoins.
I have seen first hand the rate variation from the so called official rate, if you land in a country some airport has this exchanges that trade under the official say 5% even 10% under to sell 5% to 10% over the price just because they see a business opportunity when it comes to exchanging fiat for fiat.
ever since i decided to travel with only bitcoin it has been a re leave to not deal with this type of exchanges.