Although a "blood blister" could occur with a specific trauma to the toe, in a younger, large breed dog such as yours, there is definitely the possibility that your girl has an actual tumor that typically should not be ignored.
Blood blisters will go away in just a few days. Tumors will NOT go away.
The most common types of tumors in younger, large breed dogs are mast cell tumors, histiocytomas, basal cell tumors, and melanomas. For more information on these types of tumors, please review the following links:
With that said, if this lesion persists for longer than 72 hours, than I would definitely get your girl into the vet for a visit!!
As you mention she can be a handful, you may want to ask your vet if you can get an anti-anxiety medication, like Xanax, to pre-medicate her for her trip. This can make her more calm, relaxed, and easier to evaluate during her physical examination.
Mast Cell Tumors and Melanomas are MALIGNANT tumors and should be surgically removed as expediently as possible.
While waiting to see your veterinarian, it is very important that you do not try to pop, squeeze, or express the "mass" in question, as if it is a mast cell tumor, this can cause degranulation of the tumor cells and spread of the cancer into the blood stream .Not something that you want to potentiate!!
I hope that you found this information.