You just blew the vein. This happens when you damage the vein in some way and blood leaks into the interstitial space. Pretty easy to do with a straight stick (i.e. an actual needle vs an IV catheter). Either you over inserted or the needle backed out of the vein. Using too much cuff/tourniquet pressure will increase the chance of blowing a vein. It'll heal and you can try again once the bruise leaves.
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