Taxonomy breadcrumb on taxonony block in views (by contextual filters) I have set up a block that shows up on the relevant pages that shows up other nodes with the same taxonomy terms (by means of contextual filters that take the argument from a given page).     

Can I add a breadcrumb to the header of this block view? I have Crumbs installed.     

Example Case:
This is a commerce site, and all the products are tagged by 2 vocabularies: "category" and "collection". Items that share a category tag are "internally similar" (say, all the apples), while "collection" tage are whose that the seller wants to present as similar (say, an apple and a specialized apple knife).
At the bottom of the product page are two blocks (views, contextual filters that take term argument from page) that display products that share both respective taxonomy terms with the main product on the page, and I want those to have a "taxonomy breadcrumb": not only a title with the (linked) tag  name, but also its parents (s).
I have actually  achieved that functionality by adding a views attachment to the block views.
I think this could be done with Crumbs programmatically, but this might be overkill. It would also be fragile, because it would depend on Crumbs functionality that is not really part of the public API, so could change in future versions of Crumbs.
Also consider that you don't really need the full breadcrumb, just the taxonomy trail.

Instead, you could write a custom Views header plugin. Unfortunately it is a bit hard to find documentation on that.

Or, you could alter the views header with one of the views hooks.

Or you could use the theme layer to override the views theme output for this specific view.

Or override the block template for the specific block. I think this one is actually the easiest for a starter, and for this specific situation. You can get the taxonomy term from the node, and you get the node with menu_get_object().

Have a look at https://api.drupal.org/api/drupal/modules%21taxonomy%21taxonomy.pages.inc/function/taxonomy_term_page/7 to see how the taxonomy page calculates a breadcrumb based on the taxonomy hierarchy.

(I am mentioning more than one solution here, because other readers might have slightly different requirements.)