Can I safely get through a "Blade Barrier" by crawling through with an overturned bathtub over me or does the blade barrier form inside it as well?
Not clear to me that line of effect rules apply so strictly after the spell comes into effect.
The d20srd says "An immobile, vertical curtain of whirling blades shaped of pure force springs into existence", which I interpret to mean the wall consists of a large number of blades each made of force. Nothing in the description indicates that the blades are unforming-and-reforming while the spell lasts. So the blades are each created when the spell is cast and persist until the spell's end. There is also nothing in the description that would indicate these blades can pass through solid matter.
So as long as the bathtub is reasonably tight to the floor as it passes through the blade barrier I would suggest the blades are unable to damage those under the bathtub.
The noise of blade-on-bathtub, however, would be considerable...
The spell creates a wall of force blades, which would fill its area; there is no "line of effect" relevant to its ongoing damage effect. If you believe in "rules over realism" then it's just a wall of damage and such a dodge wouldn't work - both the character and the tub and anything along would take the damage separately. In a more simulationist world, however, you could reason that such a thing should help and using a clever trick like covering up with a bathtub should help protect you somewhat from the damage.
However, there's no reason a dodge like this would make you perfectly safe. I would rule "absolutely you can use it, but the bathtub takes the damage, and given its hardness and hit points, it may split and if it does you take the rest of the damage..." There are well defined damage-to-objects rules in D&D 3.5. I'm guessing the average tub (depending on what it's made of, I assume porcelain) would count as stone which is hardness 8, 15 hp (wood or iron would of course vary this). So it's only going to take 23 points before breaking, but that could totally help - if it's a 12d6 blade barrier (average 42 points of damage), then you could get off without damage if you make your Reflex save... Of course in cases like this it's totally up to the GM whether you save for you and the tub, or save versus the remaining damage once it's through the tub, or what.
According to d20srd:
If you don’t have line of effect to your target he is considered to have total cover from you. You can’t make an attack against a target that has total cover.
Line of Effect
A line of effect is a straight, unblocked path that indicates what a spell can affect. A line of effect is canceled by a solid barrier. It’s like line of sight for ranged weapons, except that it’s not blocked by fog, darkness, and other factors that limit normal sight.
You must have a clear line of effect to any target that you cast a spell on or to any space in which you wish to create an effect. You must have a clear line of effect to the point of origin of any spell you cast.
A burst, cone, cylinder, or emanation spell affects only an area, creatures, or objects to which it has line of effect from its origin (a spherical burst’s center point, a cone-shaped burst’s starting point, a cylinder’s circle, or an emanation’s point of origin).
Therefore, anything that completely blocks line of effect blocks the blade barrier from performing attacks. The question for the DM to answer is: "Does the bathtub count as a separate object for purposes of line of effect?" If it does, one consequence is that a tightly-woven canvas sheet also counts as blocking line of effect.
Blade Barrier, as a wall, may or may not have a point of origin. Because it has an effect line instead of an area line like fireball, the last line of the LoE ruling may not apply. It however, is also not a spread, so the spread wording (not requiring LoE from the origin) doesn't apply.
The real question for your DM is "Where is the point of origin?" If each square produces its own point of origin, then the bathtub does not count as total cover. If the first square of the wall counts as point of origin then the bathtub does count. There is no clear ruling from the rules on this and no easily-findable threads on community.wizards.com.
Depending upon what the tub is made of, the blades could conceivably break or rip the tub to shreds before you had managed to crawl through their area of effect. At least that would be my take as a DM.
Wood or ceramic would be quickly or easily be broken or shredded, but you might be able to move fast enough to get through. Cast iron? Well, I would probably require one or more strength checks to move the 10' required to clear the blade barrier. Remember your crouched down on your hands and knees with very little mobility. So I would probably limit them to 5' movement on each successful strength check, meaning the tub would have to survive two rounds minimum against the damage the blade barrier causes. The hardness and hp for a cast iron tub would be Hardness 10, 30hp/inch of thickness.
Magic trumps bathtub. If the character casted a spell like Turtle Shell (dont remember where it was from Druid?) then I would allow that to provide temporary hp to the character once destroyed magic winks out you start taking damage. If player did this in my group bathtub would also damage them, shards hitting PC and surrounding players...but thats me. Blade Barrier high level spell should not be able to be subverted by a bathtub.