There are just some steps we should not take - we ought not take.
We do not need to know what happens at the beginning of life if it means that we must deliberately kill those lives we study - what we would actually be studying would be the death that we are inducing.
Take a look at this article and the one above. Do you trust these minds that can twist themselves around the abandonment of "First, do no harm"? Why would you believe that they are doing good in any form, if they are willing to do this evil?
I don't see working with blastocysts as an evil, so that falls apart for me.