What he writes may be a fair account of the changes over two millennia, but it's not of much consequence. None of what he writes says anything about whether on not any particular book should or should not be included. None of it is evidence. The whole enterprise seems to be no more than a series of arbitrary assertions — if not by Blomberg then by those he cites.
An arbitrary assertion begins Blomberg's second paragraph:
It is true that God's law and God's word last forever.
Indeed, three criteria prevailed for sifting the canonical from the non-canonical. First and foremost was apostolicity—authorship by an apostle or a close associate of an apostle—which thus, for all practical purposes, limited the works to the first hundred years or so of Christian history. Second was orthodoxy or non-contradiction with previously revealed Scripture, beginning with the Hebrew Scriptures that Christians came to call the Old Testament. Finally, the early church used the criterion of catholicity—universal (or at least extremely widespread) usage and relevance throughout the church. This excluded, for example, the Gnostic writings, which were accepted only in the sects from which they emanated.
While Catholics and Protestants to this day disagree on the canon of the Old Testament, both branches of Christianity along with Eastern Orthodoxy agree on the contents of the New. For sixteen centuries there has been no significant controversy within Christianity regarding the extent of the New Testament canon. Christians are on solid ground in affirming that these twenty-seven books belong in the New Testament and that other ancient writings were excluded for good reason.