Sandals and heel straps

It looks like the heel straps are quite wide, so maybe it's hitting your heel in tender places. My experience with Tevas is that they are either comfortable or not and any break-in is with my foot (developing a callus). Especially with the neoprene lining, which these appear to have. You might need to order from a company with a good return policy (REI, Nordstrom, Sierra), rather than trying to find them in stores.
The other suggestion, since you have a pair that otherwise work for you is to try moleskin or some type of silicone heel protector.

For questions about fit, my advice is to go into Rei and talk to the sales people. They are normally excellent with fitting shoes and boots.

I like my Keen Whispers. I have worn them numerous times on tours and the toe covers are protective if the cobblestones are slippery.

And I might add if you bought the Tevas from Rei within the last year they will exchange them.

I'm not a sandal wearer on vacation BUT I have always had problems with hiking boots not fitting so for years I would use paper tape on my heels. My brother, who was a forester, always had to tape his heels as well and through 30 years in the woods distilled down to using paper tape. It stays on as well as moleskin, comes off easier when you want it to, you can tear it instead of needing scissors, will last thru a couple of showers and it's inexpensive. I DO recommend you try this when your current blisters heal. I always used it prophylactically not after the damage was done.

I also live where I have trouble finding my size in a store so I always order from Zappos. They have free shipping and free returns...you just have to get it to UPS. If you are not sure of a size I usually order 2 sizes at a time so I can compare.

Good info about paper tape. I also don't wear sandals often but use moleskin regularly. Paper tape is surely less bulky and easier to mess with.

@Kathleen - It seems kind of wimpy compared to the moleskin but it seriously works. My brother was up/down Idaho mountainsides all day long and it held up to that kind of shearing action.

Pam, you should trade mark that "fix"! Lol, I so wish I had thought of that years ago. OP, I use Trotters. There are several styles that provide good support and look decent. I wear skirts/dresses a lot when I travel, as well as long pants. My Trotters do both pretty well. I've used them on many cobble stones! I use "Riva" and "Romi".

I also got terrible blisters from Teva sandals on one trip , when it was extremely hot and humid.
I couldn’t keep up with the bandaid use!
I had the scars for a couple of years afterward.
I think it was the synthetic material they are made of.
Now I wear Merrell or Columbia sandals, they have never given me any trouble.
Can’t tell you which styles, as they are years old now, but still great on my feet.
I like the almost suede-like texture of the footbeds too, much more comfy and less hot.