Ha! I think I finally have got the hang of Alchemy. I've summoned some demons, upgraded my altar (still don't have beacons, so no Archmage Orb yet), and made a Blood Magic Regeneration flask that is indeed much better then Vanilla for healing.
The key seems to be that you combine a Binding Agent with the Vanilla Potion Ingredient you would normally use, so Ghast Tear for regeneration, Magma Cream for Fire Resist and so on. That gets you an Empty Flask of (Effect). You can then risk adding more effects with a Standard Binding Agent, but the more you add the greater the chance it all goes boom and you lose your stuff. A single effect seems to be 100% safe however.
Then you take your flask and can boost it's power/duration combining it with Power/Lengthening catalysts, with the upper tier catalysts obviously providing bigger boosts. But even the entry level versions put together (which you can't do in vanilla) will make you a fairly strong potion flask. Each time you need to match the catalyst with whatever ingredient you used for the potion's effect, so Ghast Tear + Catalyst to boost a Regeneration flask. Catalysts don't seem to have a chance to go boom either.
Finally you fill your flask by combining it with a Filling Agent in the alchemy set. The basic agent gave me 2 uses (out of the Flask's max of 8) per combining attempt. I am unsure if the upper tiers of Filling Agents either give more uses per agent or prevent explosions when filling Multi-Effect Flasks. That will require more experiments.
All in all, crafting a Regeneration 2 (30 sec) flask was pretty easy, only requiring a bit of resources and I could have made it stronger still (and I still can I think?). Plus I found a bunch of spare agents/catalysts in chests which helped. I think a Witch farm would come in handy with Blood Alchemy, as Redstone, Glowstone, and Gunpowder are used in a lot of ingredients (especially the simple catalysts which is in everything else).
Sadly (sort of) between my 50 runes on my Altar (can craft all the things, barely) and my Bound Armor (fighting is easy) I don't really need the Regeneration potion as much anymore. Perhaps when I finally go fight the Wither it will come in handy.
Edit: You can indeed upgrade an existing flask. A single Greater Lengthening Catalyst (Tier 3) took my Regen II potion to a 3:33 minute duration per use (WOW).
The key seems to be that you combine a Binding Agent with the Vanilla Potion Ingredient you would normally use, so Ghast Tear for regeneration, Magma Cream for Fire Resist and so on. That gets you an Empty Flask of (Effect). You can then risk adding more effects with a Standard Binding Agent, but the more you add the greater the chance it all goes boom and you lose your stuff. A single effect seems to be 100% safe however.
Then you take your flask and can boost it's power/duration combining it with Power/Lengthening catalysts, with the upper tier catalysts obviously providing bigger boosts. But even the entry level versions put together (which you can't do in vanilla) will make you a fairly strong potion flask. Each time you need to match the catalyst with whatever ingredient you used for the potion's effect, so Ghast Tear + Catalyst to boost a Regeneration flask. Catalysts don't seem to have a chance to go boom either.
Finally you fill your flask by combining it with a Filling Agent in the alchemy set. The basic agent gave me 2 uses (out of the Flask's max of 8) per combining attempt. I am unsure if the upper tiers of Filling Agents either give more uses per agent or prevent explosions when filling Multi-Effect Flasks. That will require more experiments.
All in all, crafting a Regeneration 2 (30 sec) flask was pretty easy, only requiring a bit of resources and I could have made it stronger still (and I still can I think?). Plus I found a bunch of spare agents/catalysts in chests which helped. I think a Witch farm would come in handy with Blood Alchemy, as Redstone, Glowstone, and Gunpowder are used in a lot of ingredients (especially the simple catalysts which is in everything else).
Sadly (sort of) between my 50 runes on my Altar (can craft all the things, barely) and my Bound Armor (fighting is easy) I don't really need the Regeneration potion as much anymore. Perhaps when I finally go fight the Wither it will come in handy.
Edit: You can indeed upgrade an existing flask. A single Greater Lengthening Catalyst (Tier 3) took my Regen II potion to a 3:33 minute duration per use (WOW).