Two things- roan can be very seasonal, and some horses that are very obviously roan in summer might not look roan at all in the winter. If your bio for Bella is accurate and you've only had her for 4 months, then I would assume this is a big part of it. If that's the case, then don't worry, she'll go back to how she was once she sheds her winter coat again!
The second thing it looks to me like she might be a brown roan, not a black roan. You can see in several of her photos that she appears to have a cinnamon colored muzzle, which is a tell-tale sign of a brown horse. The lighter color muzzle often disappears in the summer coat, but is "always" present in a brown horse's winter coat.
“The horse is a mirror to your soul. Sometimes you might not like what you see. Sometimes you will.” - Buck Brannaman
"Nothing forced can ever be beautiful." - Xenophon