Does Alberta Have Both The Legal Authority & The Gumption To Block Oil Shipments To BC?

In a word, yes.

1 – Does Alberta Have the Legal Authority To Block Oil Shipments to British Columbia?

As explained in the video below,  ALL oil and gas products in Alberta used to be COMPLETELY controlled by the Alberta Petroleum Marketing Commission (APMC) which still exists today.  There has been no legal impediments to that process put in place since the APMC was formed in 1973, so yes the Province of Alberta can legally block petroleum shipments to BC.

2 – Would Albertans and Alberta Companies Support Blocking Oil Shipments to British Columbia?

Tom Whalen of the Alberta based Petroleum Services Association of Canada (PSAC) correctly said in an interview with CBC on April 17 2018:

“Our members are ALREADY talking about moving their people and assets south of the boarder… because our hands are tied.  …We have a ceiling that constrains us and has already put over 100,000 people out of work.  … You have to go from a battle of words to some means and measures… and send a message that this pipeline has to get built.  Its beyond British Columbia and Alberta.  The message is that we can’t do business in Canada and that we just won’t uphold the law.  This pipeline shouldn’t be at a stalemate at this point

The point being that while Alberta companies will not be keen to have the Notley NDP Government tell them where they can and can not sell their product, they will happily comply in an effort to get their products to market (Vancouver shores) for the next 20+ years.