I guess my first question would be this: Do you have any sense of what's causing the blisters? That is, does it seem to be pressure on the heel, or friction caused by too much slippage? If the former, it might be possible to have a good cobbler punch the heel of the boot out a little-leather can be re-shaped, and perhaps you can gain a half-size of heel space. If not, then the boot is too small and you'll need to replace them with a half-size larger pair.
If friction from slippage seems to be the culprit, then that perhaps can be dealt with more easily. For starters, replace the stock insole with a pair of Superfeet ($29) for high-volume boots. These soak up a fair amount of space, and may help eliminate the heel sliding. Superfeet also makes "custom" footbeds that sell for $85. These require a trip to a Supefeet dealer who can perform the fitting work (REI is one). Various heel inserts also are available to soak up some room around the heel. If your boots are one of the rare models with a leather liner, a cobbler can add a layer of leather. Otherwise, well, I remember days when it seemed like NO boots fit well, and I always was doing prophylactic taping with adhesive tape.
As for a boot that would fit better, I can't answer that. Tecnicas in general have a fairly "neutral" fit, meaning they don't tend to be low-volume or high-volume. So you need to figure out which direction your feet favor, and find boots that work accordingly. That will mean finding a store with lots of boots, asking for the best boot-fitter in the store, and spending some time trying on various models in various sizes.