It got darker because it's going grey. It is more usual for a greying horse to go darker before going lighter, than to just go lighter. This is what makes it so hard to tell what base colour a grey started as, since it doesn't just add white hairs.
The thing with grey's is each one is different and they all go at different rates and different ways. Apache went through the dark stage of grey when he was two and it was very short lived on him - by the time he was four his mane was pure white and his face was light grey with just a hint of rose. Now that he is 7 he has flea bites looking like they are showing up on his face though the rest of his body is not done going white.
I agree with Chillaa on greying. My horse was born dark, she was born a grulla. She is now 3 and is greying, but she's got dark splotches that are darker than the rest of her, and I could almost swear her mane and tail got darker near the bases. Also the dark splotches she's got are darker than what she was before