Cerys adds a "Fulgoran cryogenic crafting process" internal science pack for use with unlocking the "Fulgoran cryogenics" research, but this mod can select that internal science pack for productivity research (at the very least, the Cerysian science pack productivity research requires it)
Is it possible to a) have a blacklist specific science packs option, and b) add the cryogenics crafting process "pack" to it by default

If I understand your question correctly:

1) There's an internal science pack called "Fulgoran cryogenic crafting process" which is only used in one technology: "Fulgoran cryogenics".
2) Dependent technologies include the "Cerysian science pack" technology.
3) My mod generates a "Cerysian science pack productivity" technology which requires this internal "Fulgoran cryogenic crafting process" "science pack".
4) Since the internal pack is not intended to be used outside of one specific technology, researching "Cerysian science pack productivity" is not possible.
5) You would like a flag or blacklist that allows you to remove certain science packs (i.e., "Fulgoran cryogenic crafting process") from all productivity technologies generated by this mod.

For some reason, I cannot get the mod to generate any productivity technology for "Cerysian science pack" on my end. Most likely because it would require a combination of science packs which no lab accepts. Are you using other mods that add more versatile labs?

I just ran into this after changing mods - most notably, I added the Big Bio Lab (https://mods.factorio.com/mod/bigbiolab) which I imagine is snagging every research pack, making a lab available that can handle everything when there was not one before.

Thanks to the Big Bio Lab I was able to recreate the issue and correct it in the upcoming update.