Are all of the five aggregates saṅkhāras? Are the five aggregates all saṅkhāras -- are they impermanent and dependently originated?

I guess that "perceptions" and "feelings" are perhaps dependent on sensual contact. Is there anything else they're dependent on?

Are "form" and "consciousness" dependently originated, and what (conditions or causes) do they depend on?

Does "eye consciousness" exist when there's no contact? In a blind person, for example?

If all five aggregates are indeed saṅkhāras, then what is the aggregate called "formation" (saṅkhāra)? What is the difference in the meaning in the word "saṅkhāra" when it's used to identify that one aggregate, as opposed to when it's used to characterise all aggregates?

Is it right to call e.g. the doctrine of the "four noble truths" a dhamma instead of a saṅkhāra? Is the doctrine unconditioned and not subject to decay ... or is this a theory about Dhamma (i.e. a meta-dhamma) proposed by some schools and not others? Is awareness of (e.g. perception of) the dhamma a saṅkhāra, which depends on contact (e.g. contact between mind-object and mind-consciousness), and impermanent?
If all five aggregates are indeed saṅkhāras, then what is the aggregate called "formation" (saṅkhāra)?

Please read this answer for more info on sankharas based on Bhikkhu Bodhi's essay "Anicca Vata Sankhara". I quote the TL;DR version from that answer:


  Sankharas are "co-doings," things that act in concert with other
  things, or things that are made by a combination of other things. Ven.
  Bodhi uses "formations" or "volitional formations" as his preferred
  translation.
  
  There are 3 uses of the term Sankhara in the scriptures:
  
  
  Second link in dependent origination - when ignorance and craving underlie our stream of consciousness, our volitional actions
  of body, speech, and mind become forces with the capacity to produce
  results, including "rebirth".
  The fourth of the five aggregates - volition regarding forms, sounds, smells, tastes, tactile objects, and ideas. Also covers all
  factors of mind except feeling and perception.
  All conditioned and compounded things including mountains, fields, and forests; towns and cities; food and drink; jewelry, cars, and
  computers.
  
  
  The only thing which is unconditioned and uncompounded is Nibbana.


Are the five aggregates all saṅkhāras -- are they impermanent and dependently originated?

Yes, in all the different meanings of sankhara, they are always impermanent and conditioned (depend on other things for their arising and ceasing).

Are "form" and "consciousness" dependently originated, and what (conditions or causes) do they depend on?

MN 148 explains the arising of consciousness:


  "Dependent on the eye &amp; forms there
  arises consciousness at the eye. Dependent on the ear &amp; sounds there
  arises consciousness at the ear. Dependent on the nose &amp; aromas there
  arises consciousness at the nose. Dependent on the tongue &amp; flavors
  there arises consciousness at the tongue. Dependent on the body &amp;
  tactile sensations there arises consciousness at the body. Dependent
  on the intellect &amp; ideas there arises consciousness at the intellect.
  'The six classes of consciousness should be known.'


Meanwhile, form as one of the five aggregates refers to body, and it being conditioned and compounded is explained in MN 74:


  Now, Aggivessana, this body — endowed with form, composed of the four
  primary elements, born from mother &amp; father, nourished with rice &amp;
  porridge, subject to inconstancy, rubbing, pressing, dissolution, and
  dispersion — should be envisioned as inconstant, stressful, a disease,
  a cancer, an arrow, painful, an affliction, alien, a disintegration,
  an emptiness, not-self.


Does "eye consciousness" exist when there's no contact? In a blind person, for example?

According to SN 12.44:


  Dependent on the eye &amp; forms there arises eye-consciousness. The
  meeting of the three is contact. From contact as a requisite condition
  comes feeling.


"Contact" in this quote refers to mental contact and not physical contact. When there is physical contact between eye and forms (in this context, "form" means visual objects), eye-consciousness arises. The meeting of the three would result in mental contact.

This is only my speculation, but presumably, a blind person can see darkness or black.

Is it right to call e.g. the doctrine of the "four noble truths" a dhamma instead of a saṅkhāra? Is the doctrine unconditioned and not subject to decay

According to AN 3.134, the Dhamma does not depend on the coming of a Buddha:


  "Monks, whether or not there is the arising of Tathagatas, this
  property stands — this steadfastness of the Dhamma, this orderliness
  of the Dhamma: All processes are inconstant. ... All processes are
  stressful. ... All phenomena are not-self.


According to this essay by Ajahn Sumedo:


  Akalikadhamma means that Dhamma is not bound by any time condition.
  The word akala means timeless. Our conceptual mind can’t conceive of
  anything that is timeless, because our conceptions and perceptions are
  time-based conditions, but what we can say is that Dhamma is akala,
  not bound by time.


Is awareness of (e.g. perception of) the dhamma a saṅkhāra, which depends on contact (e.g. contact between mind-object and mind-consciousness), and impermanent?

While the Dhamma (teachings) may not be sankhara, the mental concepts built around it definitely is sankhara, which is why seeing it through wisdom (through vipassana) is needed, as compared to intellectualizing it.