Why would a rash turn into a bruise?

Rash bruise: My guess is that you are scratching the rash causing bleeding under the layer of skin which is the bruise.

Deeper damage: There may be damage to the underlying blood vessels which os called vascultis. This os a reaction pattern to drugs, or infection and my be associated to certain forms of arthritis. Ot is best ot see a dermatologist for a workup and possible biopsy fifths persists.

Bruise rash: Some rashes are called capillaritis. An inflammation of the vessels causes leakage of blood cells out int the dermis which causes a bruise. These usually occur on the legs but other rashes, some virally induced or bacterial, and diseases such as rocky mountain sotted fever. See a doctor to evaluate your rash.