Higher difficulties will yield more blues mainly because of the Nephalem Valor Buff, which is given to your character when you kill unique bosses and do not switch skills. Also, higher level gear, if they have the +% to magic find attribute, will naturally have a higher percentage to magic find that only high-level characters can wear.

Thus, I think your best bet for farming blues and crafting subtle essence is to play at as high a difficulty as is safe for your character.

The requirement level is defined by the item and every item has an internal level number.

Affix Level Minimum

In Diablo 3 every item has an internal level number, and every affix has an "affix level minimum". If an item's internal level number is below an affix's level minimum, the affix cannot be applied to the item. This prevents the incongruity of attaching extremely powerful affixes to crude, lowly items.

Source: Diablo 3 Signature Series Guide

There is no list with items and their internal level numbers so I guess they are assigned randomly for every item.

But at least legendary items also have a total affix count and bonus affixes. Here is an example:

Blackguard: LVL REQ: 48; TOTOAL AFFIX COUNT 6; BONUS AFFIXES: 2; GUARANTEED AFFIXES: Damage Bunus, Min/Max Damage, Strength/Dexterity, Crowd Control, Reduction