I'm not sure, but I think if you mod /Assets/XML/Units/CIV4Promotion_Infos.xml so that under PROMOTION_BLITZ, in the list of <UnitCombats>, you add the unitcombat (aka combat class, eg UNITCOMBAT_MELEE) of your unit. Can anyone confirm this please?
It will mean that any unit of that combat class can take Blitz, but it does have a prereq of Combat 3, which I hope is not unlocked until later.
So, if I'm right, it's not the era, it's the unitcombat. If you create your own unit in xml, you can override this by giving it the free promotion (the Repeating Crossbowman gets this promo for free even though it is not available to UNITCOMBAT_ARCHER). But that's a lot more work, and quite difficult the first few times you do it...