What type of lavender is normally used in tea blends?

If you want the “real” medicinal plant, get Lavandula angustifolia (formerly Lavandula officinalis), the true or English lavender. The flowers (medical term Lavandulae flos) are harvested when about half of the flowers on a stalk have opened up and the rest is still closed. Cut the whole flower stalks when the morning dew has evaporated, late morning to midday, in full sun. Tie small bundles, and hang upside-down to dry. Use the flowers for tea or other preparations.

True lavender comes in many cultivars that should all be usable in the kitchen. Only if you have set your eyes on a white or pink variety, it may be good to double-check just in case breeding for a specific color has affected scent intensity. (Which would mostly not be a problem in the kitchen and local climate may cause a greater variation, in my experience.)

As a head start, here’s a short list of cultivars that tend to pop up repeatedly when searching for culinary uses. Treat it as a starting point, not as absolute wisdom.

• Royal Velvet (dark blue)
• Royal Purple (dark blue)
• Little Lottie (pink dwarf)
• Rosea / Jean Davis (pale pink)
• Betty’s Blue (classic purple)
• Melissa (white to pale pink)

There are other lavenders that are commonly used in large-scale production of lavender preparations, but I wouldn’t recommend them for consumption, especially alone:

L. latifolia is more pungent, with hints of camphor - totally usable in potpourris or sachets, less so in tea.

Lavandin, L. x intermedia, a hybrid between the former two, is grown for slightly cheaper lavender products, mostly oils used in cosmetic products or household cleaners. Maybe an option, but typically considered „inferior” to the true lavender as it also has some of the pungency of L. latifolia.
However, the cultivar “Provence” is the lavender (or in this case, lavandin) of choice for cook book author Sharon Shipley, who wrote a whole book on cooking with lavender. There’s a difference between using it in teas and savory dishes, though. For the latter, some cuisines also use the soft tips and leaves, similar to rosemary.

Conclusion:
If you have room for just one or two plants, I would recommend a true L. angustifolia.