When is the earliest time that bracha over tzitzit is valid bedieved? Is there a period of sefek before that time?
In this matter there is no period of essential doubt, though there might be a period of technical doubt.
The time for the berachah over tzitzit is the time of “misheyakir,” from when a person can recognize a person four cubits away from him.
Some write that this is approximately 50 mins before hanetz hachamah, but others have mentioned longer times (up to 66 mins before hanetz hachamah), and this will also depend on the geographical location.
Under circumstances of doubt, it is best to hear the berachah again from somebody else.
See Hazemanim Behalacha 23:6.