You can safely upgrade it to +6.
I will explain why 7th affix (Create blue gem) is a controversial upgrade.
Almost always you will cast Doomed Blade only if you have red-skull alignment on the board for an extra turn when red gems will be converted to Doomskulls. In that case, the cast from Doomed Blade will be very powerful - damage to all enemies, a big amount of skull damage, Doomskulls can explode a big chunk of the board generating mana for you and at the end, it will be again your turn because you had a red-skull alignment and it was converted to 4- or 5- skull match.
However, if you have 7th affix things can go worse. Let’s say you have 1 red-skull alignment on the board for the 4-match and you are ready to cast Doomed Blade. That created blue gem from 7th affix can destroy your alignment and you won’t get an extra turn. The chance of it is not very high, if you have only 1 alignment it will be somewhere ~6-8% depending on the board if my calculations are correct, and if you have more than 1 alignment it won’t happen. But if it will happen you will be screwed - the board will be populated with Doomskulls and it will be an enemy turn - you have a good chance to lose in that case instead of destroying the enemy team.
As you can see 7th affix for Doomed Blade is a downgrade in reality and you will understand it very well after your first lose because of that affix.
8th affix for Doomed Blade (Freeze the first enemy) is quite good and 10th (Drain 2 mana from blue enemies) is decent and you cannot get them without the 7th.
It is your choice - do you want to spend a lot of gems to get forge scrolls to upgrade Doomed Blade past 6th affix to get 8th or 10th affix and accept the fact that bad RNG from 7th affix can cost you games? or you can spend less, stop at 6th affix, don’t get that freeze and drain mana but you will be sure that your red-skull alignments won’t be destroyed.
There is no 100% correct answer to that, it’s why I said it is controversial.