Blessing of Belzenlok gives +2/+2 and if it is cast on a Legendary creature it also gives Lifelink. But it never recognizes Historic/Legendary creatures.
Have tried a few sofar and none gets the extra lifelink.
Tetzimoc, Hallar, Radha and Yargle have been tested with Vraska in Across Ixalan today.
//Edited Title -Tombstone
Still bugged. 
Wish I would have seen this before I put it in a deck this weekend. As it does not work as OP stated.
This is so frustrating. This and other spell cards just useless because the ability they provide doesn’t stick.
That being said, some other cards are bugged like this, too..
https://forums.505go.com/discussion/74296/requirement-ability-buffs-not-sticking#latest
This card is still bugged and no official [investigating] label
Hi everyone, the team is aware of this issue, and it is currently scheduled to be fixed in update 2.9.