Say I Fortify my Blade skill from 40 to 90. For the duration of the fortification spell (or if it's a constant increase from an item), will I increase my Blade skill as if it was 40, or will the learning be slower and as if it was 90?

Also a question on the Block skill - for the rate of learning, does it matter if I block unarmed, with a sword, or with a shield? In other words does the rate of Block learning depend on what you block with.

First question, from the http://uesp.net/wiki/Oblivion:Fortify_Skill

Fortify Skill normally has no effect on the rate at which you gain experience in that skill. For example, if you are level 12 in Conjuration, it takes two spell casts to advance to level 13. With +10 Fortify Conjuration, it would still take just two spell casts to advance from boosted level 22 to boosted level 23 (whereas four spell casts are necessary once you naturally reach level 22).

Second question, again from the http://uesp.net/wiki/Oblivion:Block:

Your block skill increases by 1.25 experience points each time you successfully block an attack, whether using a shield or a weapon, regardless of the amount of damage blocked.

And from further down the same page:

It is faster to gain experience barehanded (no weapon equipped) than with a shield.From level 0-50, blocking barehanded has no recoil for the opponent, allowing them to attack more often. At level 50, blocking barehanded will occasionally provide recoil, but still less often than with a shield.

Thank you very much for the help

But remember, if you fortify blade, you will do more damage, which means you will kill opponents in fewer hits, which means you will get less experience for killing that opponent. Fortifying a skill does not directly reduce the rate of skill progression, but the effects of fortifying a skill can indirectly affect it, depending on the skill and how you use it.

One more point, there are two ways of "artificially" raising a skill, one is with temporary fortifications and the other is by adding an "ability" that will raise the skill.

As mentioned above, fortifications do not affect the amount of experience needed to raise a skill.

However a skill raised by an ability will change the experience needed. In many cases an ability is permanent. Such as racial abilities (you can see them in the magical effects tab). But in some cases an ability can be added and removed. For example, the alchemy lab at Frostcrag, the Skeleton Key, or Scales of Pitiless Justice. They increase skills via abilities, and will require more experience to raise a skill when they are active.

For example, with the Skeleton Key ability increasing the Security Skill. You will have to raise more tumblers to increase Security if the Key is in inventory. One trick is the raise tumblers with the Key in inventory until the Security bar is almost full. Then you can drop or store the Key and sometimes you can raise Security twice in a row with by just raising two tumblers. (I recommend retrieving the key when that is done).