Its a void in the bloodstain. The photo has been manipulated to make it appear brighter than the rest of the fabric.
There is no point in falsely manipulating a photo for which negatives exist. As explained, any filter is to assist in clarifying the position, shape and design of the object in question. Like I said, if police enhance footage of a parked van to clarify the license plate number, it is the same thing. The object is present it just needs clarifying.