I think that's a good idea to give not just dragons, but all solos their own weakness. I would make it not based on the species but rather the individual. For example, an obvious weakness for brown dragons would be food, but that's too obvious. Maybe one brown dragon has a particular place in his heart for camel meat, while another will stop fighting at the promise of sweet tasting desserts.
For the blind spot thing, an elder or ancient dragon could have received a cut on their eye at some point in their life, so it can't see in a particular direction. But I wouldn't have, for example, all blue dragons have the same blind spot, as that would get predictable and easy to find out beforehand. The PCs should find the solo's weakness in the middle of the fight, around when they get the dragon bloodied, which is when the dragon starts getting more dangerous too.
