This is not so much an answer, but I want to generalize Colton's point in this way: I can understand there might be hidden clues and meaning twitching in a riddle. However, a riddle should be strict enough to not allow ALMOST ANYTHING to be an answer. Here I present the answer
"Any tool or thing that can be used as a tool, thus any thing (non-living)"
Darkness makes it harder to work with me.
It is harder to work with any thing during the night, 'cause you can't see.
For it blinds your eyes but doesn't effects mine.
See previous point
For I become the slave once and for all
When you use a thing, you could refer to it as a slave
until my main part is no more subtle.
It is easy to define a "main" part for each thing, and it is in most times philosophically valid to describe it as "subtle" when it's working, and not when it's not, since when it's not useable anymore it will exhibit defects.
That's how my life sequence is joined.
Considering "sequence" is "a series of events", it could be widely synonymous to time, and the "joined" part just means "time goes on"
Thus, I think this riddle should REALLY need to be more specific.