Does an ELC affect juniors eligibility? Zacha signed one and he's still in juniors this year.
Nope.
Blackwood can stay in juniors for the rest of this year and next year and his contract will simply "slide" to the next season. So he can debut as a pro in 17-18 and still have three years left.
"What is an Entry Level Slide?
If a player who is signed to an entry-level contract and is 18 or 19 years of age (as of September 15 of the signing year), does not play in a minimum of 10 NHL games (including both regular season and playoffs; AHL games do not count), their contract is considered to ‘slide’, or extend, by one year. For example, if a player signed an ELC for three seasons from 2015-16 to 2017-2018, and their contract slides, their contract is now effective from 2016-17 to 2018-19.
Players who sign at 18 years old are eligible to have their contracted extended for 2 seasons. This extension does not apply if the player turns 20 between September 16 and December 31 in his signing year. Signing bonuses do not slide, and are paid to the player regardless of a slide, this causes the annual average of the players contract to change, and therefore the cap hit decreases for this player."
I think teams and players both find it beneficial to get the contract done early sometimes, even if a player won't turn pro for a couple seasons. The player gains security from not having to worry about regression in play or an injury causing him to go unsigned, while a team can sure up their financial projections for the coming years.