A middle-ground alternative might be to say, "Hey, delivering blockchain/ICO emails costs usn% more than your average email. Therefore, we will still deliver them, but at n% the usual price."
It's a little like UPS/FedEx charging more for hazardous-material shipping.
If n is large, it will have a similar effect without feeling like MailChimp is out to censor cryptocurrency.
Except it's not a pure cost thing. Delivering blockchain/ICO emails impacts the deliverability scores of everyone else routed through those same domains and IPs. It's incredibly difficult to pin that to a number, because it's hard to quantify in advance how much of an impact one customer's blockchain emails will have on your IP/domain reputation.
The only way to make it a pure n% cost is to isolate the individual customer or all blockchain-email customers to a dedicated set of IPs/domains that can have their own (probably: really shitty) reputations. At that point, it's highly likely you'll go through all that effort just to take money from people and still not be able to deliver their email due to catastrophically low reputation infrastructure.
Refusing to accept the impact to your other customers of a hot spam topic is not "censorship". People really need to stop misusing this word. Mailchimp is a business, and has every right to take reasonable steps to protect their customers. It's already pretty common for porn and get-rich-quick businesses to be refused by these services, largely for the exact same reasons.
The problem is that the cost of Blockchain / ICO spam does not just fall on Mailchimp as a company, it falls on MailChimp's customers, who have to suffer through reduced email deliverability (for the same cost they are already paying).
Other than that, MailChimp is a private company, not the government, and can damn well serve who they want.
But why should they? Doing that brings very little upside for MailChimp and a lot of potential downside if Google decides to blacklist MailChimp's entire IP range.
And then they have to devote engineering / moderation resource to routing emails to the correct IP shard.
Mailchimp has offered dedicated IPs for years - the code and operational processes around that are already established. Gmail also rarely blocks at the IP level, especially for an established provider like Mailchimp.
Ultimately, this is first and foremost a policy decision around the extent of fraud in ICOs generally, and secondarily related to many ICOs' lack of adherence to Mailchimp's anti-spam standards.
It's common for businesses to turn away customer segments that could be antithetical to their ability to service the rest of their customers. For example, many credit card processors will turn down businesses with higher rates of fraud, like strip clubs. That keeps everyone elses fees lower and lets them operate a more efficient fraud department.
On top of that it's hardly like Mailchimp has some sort of monopoly. There's dozens of ESPs and turnkey email platforms that will welcome blockchain business... they just don't have Mailchimp's inboxing rate.
It's a little like UPS/FedEx charging more for hazardous-material shipping.
If n is large, it will have a similar effect without feeling like MailChimp is out to censor cryptocurrency.