Sometimes when touching something in the yard, a blood vessel in the middle finger of either my left or right hand will expand suddenly, leaving the area swollen and blue or purple. It is accompanied by severe itching during the expansion phase, usually several seconds. The discoloration lasts several hours, only to disappear without a trace.

My first thought was that this was an allergic reaction, but there are absolutelly no changes to the surface texture of the skin and nothing has pierced the skin. The itching never lasts longer than the initial expansion phase. I also never get it on any other finger.

Touching something --- or perhaps working on gardening for a while, leaning over, strenuous yard work?
You could have something pinching vessels in each wrist that effects you when doing a particular bending of the wrists and while bending down or kneeling.
Are you on any medications, blood thinners, beta blockers, anything? How old are you, how's your BP etc?
I was on something years ago that made both hands turn blue in a hot shower, but not in a sink full of hot water doing dishes. I never figured it out but I think it had to do with my hands being down at my sides, and being on a drug. It went away when I went off the drug.