This is a different expert. Sorry that you had to wait for such a long while but your question was just opened up to other experts.
Now bruising on the arm or for that matter anywhere on the surface of the body can be due to a number of things. Since your blood work was normal this can be due to problems such as a insect bite. Considering that you also havea scab on one of them could indicate that these two bruises are due to an insect bite. If this is the case then this would be best treated with the help of antihistamines and if there are signs of infection like redness, swelling etc then you would need antibiotics as well.
Now if these bruises appeared spontaneously, then other underlying causes would have to be ruled out. Sometimes simple blood work wouldnt pick up on bleeding disorders which are functional in nature. This would need to be determined with help of tests like PT/APTT/INR as well as bleeding time and clotting time.
Now it is also possible that you could have developed these bruises due to you hitting your arm inadvertently, with increasing age the capillaries are more easy to damage with the slightest trauma as well. Once youi go above 40 changes start to occur in the capillaries of the body. These would usually disappear on their own after a couple of weeks.
Another cause could be medications. If you have been on aspirin that again can cause bruising of this sort to appear. Additionally use of topical steroid creams can also cause this sort of bruising to appear as well.
Now it would best if you could attach a picture, here is a link on how to attach a picture here
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