So, it looks like blueberry leaves are not toxic to humans, rats, or dogs. The leaves have been ground up, ingested, and tested and made into teas even. But fertilizers and soil acidifiers can be a problem. I couldn't find any information about birds specifically - and I know birds can have certain sensitivities that other species do not. Probably more research is needed, and we may not be able to find anything.
I did find this interesting link, which says that braken fern is toxic to birds! We have that growing all over our property, so you know what I'm going to be doing this spring!! But I don't think I can do a thing about the buttercups that carpet the property.
https://birdhism.com/blogs/news/common-bird-safe-house-plants
For butterfly bush alternatives here's a potential list:
Abelia x grandiflora, glossy abelia
Ceanothus americanus, New Jersey tea
Cephalanthus occidentalis, buttonbush
Clethra alnifolia, sweet pepperbush
Cornus spp., dogwood
Kalmia latifolia, mountain laurel
Lindera benzoin, spicebush
Salix discolor, pussy willow
Spiraea alba, narrowleaf meadowsweet
Spiraea latifolia, broadleaf meadowsweet
Viburnum sargentii, Sargent's cranberry bush
I'm unaware if any of them are toxic to birds, we'd have to research that after you decide if you like them visually.