[Matrixbeast]

I didn't feel that OOO had a slump, personally. It did slow down, yes, but the pacing was well, the Medals were still shifting, so fights were always kept relatively fresh, and most of the actions the villains took were relevant to the plot. While there were certainly filler episodes, OOO never felt dull or boring to me.

Blade is a different case. It's not a slump, it's just that the first arc of the series is about Tachibana/Garren, rather than the titular Blade or the popular Chalice. So it feels like Blade is more of a set piece to Garren's story while sewing the seeds to Chalice's story at the start, which is why it feels slower or less interesting.
I personally liked it, since Garren's relevance drops off once Chalice and Leangle hit the scene, you still know who Garren is and what all he's been through during these events as the series starts the real ride.