My name is ***** ***** I’ve been involved professionally with dogs in the health and behavioral fields for over 18 years. It will be my pleasure to work with you today. I'm sorry your question wasn't answered in a timely manner.
Dog tears and saliva contain an enzyme that causes a chemical reaction on the hair turning it red, then brown and eventually black as it builds up. Of course, your dog might also have dark skin under the tears from a yeast dermatitis under the hair from the tear buildup. There are many products on the market to remove this. They are marketed as Dog tear Stain Removers. The following site has a good chart that discusses these.
The website below has a different approach suggesting altering your dog's pH levels to prevent the staining which you may wish to look at.
There is also an antibiotic that your vet can prescribe called Tylan that has been shown to be useful to stop tears and saliva from discoloring a dog’s coat. You will want to ask your vet about this.
If the skin under the hair is darker, then the constant build up has allowed the area to stay wet and your dog may have developed a yeast infection. You will want to clean the area real good, being very careful of the eyes while you are doing this. Trim the hair back and try some miconazole cream which may provide some relief until you can have her seen by your Vet.
When you have her seen, you might ask your vet to check that the tear ducts are not blocked leading to excessive tearing as well.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.