I'm sorry for your loss. It must be hard on you and Danna.
Cats absolutely feel loss and grieve. There's a danger in projecting too many "human" emotions onto our cats, but there's no doubt that cats feel the loss of their animal companions.
I had to euthanize my German Shepherd two years ago, and my cat has never been the same since. They grew up together and were best friends.
Look at it this way - the heart is infinite. It can stretch to accomodate new loves for new pets. But when we lose a pet, the canvas of the heart is cut. There is a hole in the shape of our beloved pet. No other pet will exactly match that shape, no matter how hard you try to shoehorn them in. You cannot "patch" over that hole with another pet either - the patch won't be the right shape! I know my example seems a little corny, but it's how I have felt through all my pet losses.
Your heart will grow large enough to love a new pet. The new pet will never, ever be Blackie's replacement, but he or she can be a fitting heir to the love you had for Blackie :}
As for getting a new cat, I would say absolutely do so if YOU feel emotionally ready. Remember that cats aren't human - they do grieve, but you don't have to "give" Danna a set amount of time to mourn. Animals adapt quickly to new situations, and even though she won't ever forget Blackie, a new pal could give her a new lease on life. Remember, SHE is used to having another cat around. Suddenly being solo is probably more stressful than getting used to another cat.
How old is Danna? If she's not too old (if she's under ~12 years old), you are fine in getting a new cat. I'd actually recommend a kitten. A new kitten can re-energize an adult cat, and can distract an older cat from grief (in a good way!)
A kitten, also, has a good chance of being somewhat moulded by the older cat. A kitten is not set in its ways or personality the way an older cat might be. If you adopt an adult cat, it may have personality quirks or habits that Danna can't adapt to. It's not impossible for an older cat to be introduced to another adult cat, but it seems like a kitten would be easier on everyone.
Good luck! Let us know what you decide to do :}