I didn't have blue finger but a similar thing happened to me. The Rheumatologist was baffled and said all he could suggest was that it is Vascular. It came out of the blue and disappeared just as quick. It looked like I had been eating beetroot ha ha.
I have had R/A for 12 years later about 6yrs ago I was diagnosed with Fibromyalgia.
You weren't doing something that would have restricted the circulation in your wee finger? My hands go like this if I am carrying plastic carrier bags.
The bizarre thing is I was just sitting doing almost nothing. It happened again at the base of my thumb a big purple blob. It isn't painfull just unsightly. Who knows. Such is life with a condition. Ha ha.
I used to get these purply-red blobs, usually it seemed near joints that were flaring. No-one seemed to know what the cause was. However, since I have been off methotrexate I haven't had them again, so I'm wondering now if it some drug reaction to MTX.
My youngest had something similar first time he ate dairy. It looked like he'd bruised the inside of his hands. Doesnt eat cow dairy luckiky he can tolerate goat.
I have had this a few times usually when I'm in a flare, but do have bright red palms all the time, this is also a puzzle does any one else have this? ,
Cold and pain go hand in hand
Steroid injection in hands?
Joint replacement surgery on hands
Wrist and hand operation. 
unbearable pain in wrists and hands
