One general rule for leather is that leather will only get darker, and never lighter in colour, unless it is painted - which I do not recommend for Asso straps. The black line stain is permanent.
General maintenance: I suggest a bit of leather conditioner - beeswax leather balm - but only if you see that the leather starts to dry out. If you wear the strap regularly, there should be no need to use conditioner because the leather will absorb the oils in your skin/elsewhere and self-maintain. However, if you use the strap underwater, the leather will need conditioner.
If the leather gets saturated with water, let it dry out slowly, and never in direct sunlight or heat source.
Leather watch straps basically just darken up over time/get scratched, and that patina is a major role in the overall mojo of your watch. Enjoy!