So you rather go from having 3 possible names to NN, though she might have been given the birth name Blæja just because you don't want to guess a name? I rather stick to any of the name mentioned and choose the most likely because that is how we find her in the sources no matter what name she ever had. Well I can live without any surname, but it seems ridiculous to write N.N.
When it comes to Anne Brannen's declaration of no habit to use last name for daughters, you forget that initially the Norse had their habits with them, it takes time to change and it doesn't happen over one night. I'm pretty sure there also existed runestones even in UK, at least, it's on them we have find the earliest proof for patronymic for daughters here and one of the Scottish language were Norn, similar to Norse, it didn't disappear over one night but instead developed into Lowland Scots.
When you mentioned Anglosaxare, you may also know that they were there earlier, but Northumbria lies in the more northern parts and in this case we are in the first quarter of the 800's century, not 500 years earlier, and this later time coinciding with the vikings attacks and settlement in this areas and cultural influences are usually bi-directional, whatever which cultural habits that wins in the end, it is likely difficult to determine exactly what was applied at all locations at a given historic time unless just what the scant findings tells us about, so to state some kind of historic Anglo Saxon overall one common way and practice feels very uncertain.