Nin/blm is affected because blm gives magic attack+ trait which affects nin's elemental magic attacks. However, summons aren't really attack magic, so that is where the question arises. Most smn's on ffxionline.com state the blood pacts damage is based on the summon, summoner's lvl, and maybe summoning skill. I don't remember ever seeing a smn comment about +magic attack items (like the earring) having any effect on their attacks. Even if it did, the effect is small.