Black Clover's current timeslot is best suited for long running series, though a lot of recent series haven't been much longer than 2 cours. Its aired a lot of western series recently as its been airing the Lego series since April and it aired Avengers Assemble before that. Bleach was only in this timeslot from its start in Fall 2004 to the end of Summer 2005 and Pokemon was only in this slot for half a year back in 1997. The times are entirely different though so you can't use their numbers as a gauge for BC's success. In recent years, I've only been able to find Majin Bone's, which aired till the end of Winter 2015, ratings and it was on average anywhere from 1% to 2%. Series after that have almost no information available besides a few numbers tossed around and most of those are in the 2 percent range. Its still entirely possible that BC is able to get high ratings but the last time that was seen was with Youkai Watch, which airs in the same time but on Fridays. The show before Youkai Watch didn't get anywhere near the same ratings as it did soon after it started, averaging at least 4% in 2014 and going even higher.
In terms of boost however, it can still happen as Haikyuu didn't get high ratings but still got a massive boost. But, it will need close to a miracle to get anywhere near 500k from its current 150k-200k. Magi is the only series in recent memory that has gotten such a boost and it was on a much more mainstream timeslot.