ToME: the Tales of Maj'Eyal

Is there a viable build for the Summoner class to use the Blighted Summoning prodigy? How many points should I expect to spend on my magic stat, and how much can I expect to get from my equipment?

Sure. While the prodigy itself is at best second-rate for summoners in my opinion, the class is forgiving enough to go that route and still win. If weird summons are your cup of tea, it certainly is a fun prodigy to play with.

If you didn't join Zigur, you shouldn't have a problem finding enough gear to not have to invest a single point in Magic and still qualify for the prodigy. On the other hand, if you want to get the most out of Blighted Summoning, you should definitely max your Magic stat third, since your summons inherit it as a bonus to theirs, thus boosting their own spellpower. It'll make their new abilities more powerful and harder to resist.