My daughter was playing w/a friend & got a fingernail to her gums. it looks like a blood blister on gum line. should i take to dentist?

Not necessarily.: She must have stopped Bleeding. The Mucosal lining is very delicate and there was a laceration and now that skin is stopping the bleeding. As long as she is not in severe pain and not continuously bleeding she will be fine and will heal. Give something cold toeat and drink like Popsicle and Ice Cream. She will be happy and the area will not be irritable

In some cases: If she develops redness, pain or pus around where she was injured then she should see a doctor or dentist. If it is just a blood blister, then don't open it and it will heal on its own.

Mouth rinse: Using antibacterial mouth rinse like Listerine several times per day should help. If it does not get better in a week you should take her to your dentist.