This happens to most of us on occasion. The insulin is injected into body tissue directly and the blood is a reaction to the surface puncture. I wouldn't worry unduly over this as the insulin is absorbed by the tissue and eventually carried into the bloodstream. Unless it is more than a bead, I wouldn't be concerned. Many has been the time I've blotted a shirt with a dot of blood post-injection. I can, however, see your point, but I am sure that it is fine and normal. As a post-script, try varying the angle from 90 to 45 degrees, it's less painful and is less likely to make you bleed, I find. Pinch gently the injection site until the needle has broken the skin, then release. Also, if you don't already do so, as it was not stated, always use a fresh needle tip. Thay way the platelets, however few, will not cause infection or offer resistance to insertion, pre-inject. A sterile needle every time and the 45 degree angle lessens the risk of bleeding and/or bruising afterwards, as there is no infection risk.
Give it a go, if you don't already do this.
Best of luck and hope some of this is useful. Let me know if it does in a day or two if you want to.
Mart