So the standard blending formula that I use is:

((Vn * Gn) + (Vn1 * Gn1) + (Vn2 * Gn2) ...) / (Vn + Vn1 + Vn2 ...)

Mathematically, what I would expect in your case right when you did the blend would be Batch A having a ABV of right around 9%, and batch B having an ABV of right around 12.4%

That leaves us with ((4.75 * 9) + (4.75 * 12.4)) / (4.75 + 4.75) for ABV:

Which equals 9.5L of 10.7% mead.

And ((4.75 * 1.084) + (4.75 * 1.000)) / (4.75 + 4.75) for gravity:

Which equals 9.5L of 1.042 gravity mead.

Now the tricky part is that gravity readings with alcohol already present don't mean the same thing as those without. A 1.042 mead with 10.7% alcohol is not the same reading as the same liquid without alcohol. Alcohol is less dense than water and so the actual gravity for the purposes of an ABV calculation should actually be higher, I'm pretty sure most ABV calculations need an ethanol free first reading and take into account the density change with regards to the final reading.

Now here's where I'm guessing... I'd love to learn more about this so if anyone has a better answer please learn me.

So in our mead our alcohol is 10.7% of the total volume. That means that for 10.7% of the volume we need to correct for the density of the alcohol to get a more accurate "OG" number.

If we account for ethanol being around 79% as dense as water (or water being 21% more dense) we should be able to figure it out.

So if we correct the entire thing by 21% we get (42pts * 1.21) = 50.8pts.

And then take 10.7% of that we get (50.8pts * .107) = 5.4pts which should be our correction? I don't honestly know... I'm shooting from the hip here, but let's go with it.

So your corrected OG is 1.047. If that then fermented down to 1.020 you've created around another 3.5% ABV which means that all in all, right now, your mead is sitting at about 14.2%.

Maybe!