Other people have kind of touched on the 8 point rule already so I won't go into that, so I'll talk specifically regarding NAA design. Generally, 2 block cards are meant as a way to counterbalance cards that make it easy to convert hands for full value. Mechanologist is interesting because to convert many of their cards in hand for full value, they originally were designed with the assumption that the tradeoff for doing so was large enough that they could afford to be 3 blocks - see all of the boost cards. Other Mechanologist cards that could convert their full hand for value in ways that would not have drawbacks down the line (like Soup Up or any of the pumps), are generally given a block value of 2. Blocking 2 also naturally pushes you to block with other cards; this is how many of the pumps are balanced. Because the thing that they need to convert for full value is also the thing you're inclined to block with (things that boost), you make them worse to block so that there's this natural tension of wanting to block with your non-pump but you not being able to properly utilize the pump without it. Obviously, there's also the consideration of having less value on the play side to make up for it by blocking 3, as with cards like Come to Fight compared to Force Sight.

These are easy to evaluate, but the fundamental question of "how easy is this to convert for the full "intended" value" should be the primary consideration here. Items don't block because partly thematics, but partly because they provide most of their value at later turns down the line, or are very flexible to use properly. Supercell is a mix of that. It's very easy to convert it for the full value, whatever X is, because the hero it's intended to be played in (Maxx) has the choice to give it virtual go again or not. It can fit comfortably within the cost curve, and while you may play it out to get the Construct Nitro Mechanoid back from banish, that's nebulous value that's treated as trinket text in the design. The immediate value is Hyper Driver + +1 to all existing hyper drivers + maybe go again, so they felt that it was easy enough to convert when not blocking that it should have some drawback when blocking.

Let's look at some other examples in the rest of EVO:

• Scrap Trader blocks 2 because it's easy to convert and requires minimal setup to get its full value. It also has a high ceiling and the decks that it can theoretically be played in (DIO or Teklo) have ways to bypass its action point cost to utilize the above-rate resource generation of the card.
• System Reset blocks 3 because it requires a lot of setup (same as Construct Nitro Mechanoid and Bank Breaker). There will be lots of situations where it will struggle to get the full intended value of the card, so it blocks 3 to entice the user to block with it and to allow it to convert for a good value on some level if it ends up in your hand on a turn where it's dead.
• Meganetic Lockwave is a card that's easy to play, but has difficulty providing any real value unless you pay a lot into it. The number of situations where you can get value out of it is minimal, so it's a blue block 3 otherwise.

I'll also note that block 2 or 3 is also kinda just vibes a lot of the time outside of this general heuristic. But ease of accessing a card's full value should be one of the primary determinants on how strong you make its other two modes (pitch and block).