What would be the best way to repair these without it looking too obvious?

Whatever you do the repair is still going to show. My mother used to sew patches on pillow cases all the time. We were Shabby Chic way before its time.

Instead of trying to hide the patch, are the holes in a place where you can add "bling". Can you patch the holes with something decorative, and make the pillow cases look 'cute' instead of 'patched'?

Also, I have read the if you sleep with Benzoyl peroxide on your face that it can damage fabric.

Might a drop of bleach have fallen on the cases where the holes are? I had that happen to a couple of kitchen towels, and it might have been the cause. (I don't use chlorine bleach anymore...). I was also suspicious of my washing machine...run your hand gently around inside all the edges of your machine and see if there is a sharp bit somewhere. I had a little sharp plastic point on the rim of the basket, and nipped it off with nail clippers. After those two changes, no more holes.

Well, after reading one of these responses, I think I might have the answer to what may have caused some of the holes. I suffer from somewhat frequent nosebleeds in the winter months, and have bled on these pillowcases. I have put drops of bleach on the spots when I need to, since nothing else seems to work. I'm guessing this may have weakened the fibers and caused the holes, though months later.
The effected sheet sets are white, so that is the only reason I have used bleach. At this point I have nothing to lose, so maybe the stitch witchery idea is best. Unless someone else come up with a better idea..

You might want to try plain ol hydrogen peroxide on blood stains. just pour it thru the stain and it removes it. Even does pretty well on dried blood. This is a trick nurses use on their scrubs. I learned from a friend of mine. Now I keep bottles of hydrogen peroxide next to my detergent to use as necessary.

First, a suggestion for removing blood (your own): the most effective solvent - even for dried blood - is the saliva of the same person. Just spit on it and let it soak in and then rub after it softens up. it may take a few doses but it always works. (Saliva of the same person is best; then same blood type, then same species, but when you get down to cross-species, it's not as successful at least not in my attempts on cat or dog blood.) Hydrogen peroxide is good, but saliva is completely non-damaging and an excellent first step. Generally, I don't need anything else. I find that bleach is insidious - you successfully shift the stain, and all seems well, but actually you've damaged the fabric and a hole is just waiting to happen.

Now, for the mends, here's what I would do: With closely matching thread sew a row of very small backstitches about 1/32 inch from the torn edge (or more if you've a lot of fraying) along the edges of the tears or holes. This stabilizes the edges. Then darn over the hole, either by hand or with a machine. When carefully done you'll have the visible equivalent of a bartack/buttonhole stitch over the hole. And the mend will seem very inconsequential after the case is ironed. You'll never feel feel it in use.

These types of mends on cases are very common in my household because my cats often jump up on the bed and the down pillows seem a little unsteady to them so they extend their claws. If I don't catch it right away, then a little tear gets bigger in the wash.

Molly~

The hole in my vintage pillowcase is just a litte too big to darn over. I'm thinking of making an "applique" of white cotton material on both the right and inside of the fabric, sandwiching the hole between the two patches. Hope it works!