From meeting identical twins in my lifetime, yes I’d say they are quite a blueprint for an organism. A blueprint doesn’t mean things are exactly the same, yes there are environmental factors, but there is a lot going on there that is the same. Identical twin studies done with twins separated at birth and experiencing different environments will show you that.
Depending on what exactly you are studying, there is a much more important environment, that has dramatically different outcomes even for the same genes: the uterus you develop in. If you implanted two genetically identical fertilized eggs into two different women, you'd see a much larger difference, since the mother's body has a significant active role in controlling gene expression, one that's often forgotten about in such discussions.
And even then, it is far from clear how much of the similarity comes from the matched genes, and how much from the original egg cells having fissioned from a common ancestor, independently of the contents of the nucleus. We know the cell itself actively seeks a goal, on its own.
As that first cell divides again and again to generate the trillions of cells in your body, mutations happen in the hundreds and thousands. Even though they started with all the same genes, they certainly don't stay that way as you grow.
https://www.gu.se/en/gnc/what-have-twin-studies-taught-us-ab...