Blue summons can be captured at 50. Purples require 53. You have to double-click the rune to equip it.

I highly-recommend you go around and hit every summon once, because the First Time Summon Capture Bonus is half of their required fragments, plus two runes (lvl 1 if blue, lvl 2 if purple). That way, even if you get a summon from the summoning scroll random thingit? All the fragments you already have get converted into summoning tokens which are used to buy more runes, and you've already got a *load of runes besides! The stat bonuses are always just passive stat bonuses, because the summoned animal relies on your Ninjutsu and Taijutsu for damage.