Blood Aspirant — ruling 1

Blood Aspirant's first ability is a triggered ability, not an activated ability. It doesn't allow you to sacrifice a permanent whenever you want; rather, you need some other way of sacrificing permanents, such as its second ability.

Blood Aspirant — ruling 2

If you sacrifice a permanent as part of casting a spell or activating an ability, Blood Aspirant's first ability will resolve before that spell or ability.

Blood Aspirant — ruling 3

Blood Aspirant can be sacrificed to pay for the cost of its own last ability.

Blood Aspirant — ruling 4

If the target creature is an illegal target by the time Blood Aspirant's last ability tries to resolve, the ability won't resolve. The creature can block as normal. If the target is legal but not dealt damage (most likely because of a prevention effect), it can't block this turn.

Blood Aspirant : règles, fonctionnement et interactions (MTG)

Blood Aspirant is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Whenever you sacrifice a permanent, put a +1/+1 counter on this creature. , , Sacrifice a creature or enchantment: This creature deals 1 damage to target creature. That creature can't block this turn.

How it works : Blood Aspirant

Original English text — Wizards of the Coast / Scryfall

#1January 24, 2020· Wizards of the Coast
Blood Aspirant's first ability is a triggered ability, not an activated ability. It doesn't allow you to sacrifice a permanent whenever you want; rather, you need some other way of sacrificing permanents, such as its second ability.
No other card is named in this ruling.
#2January 24, 2020· Wizards of the Coast
If you sacrifice a permanent as part of casting a spell or activating an ability, Blood Aspirant's first ability will resolve before that spell or ability.
No other card is named in this ruling.
#3January 24, 2020· Wizards of the Coast
Blood Aspirant can be sacrificed to pay for the cost of its own last ability.
No other card is named in this ruling.
#4January 24, 2020· Wizards of the Coast
If the target creature is an illegal target by the time Blood Aspirant's last ability tries to resolve, the ability won't resolve. The creature can block as normal. If the target is legal but not dealt damage (most likely because of a prevention effect), it can't block this turn.
No other card is named in this ruling.