Another thing comes to mind: historically, heresies tend to have a heresiarch, or sect, as one of the main disseminators of the error. In the case of BoD, there doesn't appear to be an individual or group that is responsible for disseminating the error, as you find with many of the larger heresies, such as Lutheranism or Arianism. Yet, according to the claims of "Feeneyites" and "Dimondites", this particular error is one of the most widespread in Church history.
Who is it that we can look to as the root of this error, if it is to be considered as true heresy? Given that Lad has pointed out that there are varying definitions of Baptism of Desire and Blood, there is almost an impossibility in tracking the beginning of the error to an individual or group. One could point to misunderstandings of Ss. Augustine and Ambrose; yet another the liberties taken by theologians in writing the Tridentine Catechism; or even the musings of Ss. Thomas, Alphonsus, and Bellarmine on the issue. Or do we pin it solely on the Modernists, even though it has pre-dated them by at least a century or more?
Now, in the case of invincible ignorance-BoD (I think coming up with a new, distinct term is in order), one could point to it stemming from Pelagianism given that it attributes merit to the actions of Man rather than the workings of Grace. But even then, it doesn't necessarily rely on the individual merit of Man, as it lies solely in the abundance of Grace God pours out on individual men by His Mercy. Yet, not all of these same proponents define it this way. As many of the more modern traditionalist clerics tend to view it as a speculation on the bounds of the Church Herself. Do the bounds of the faithful reside solely with those visibly in communion, or is it further than that?