Prospects in that space are likely to be technically capable, though could just as likely be entrepreneurial hopefuls. Either way, they'll likely consider outright pitches spam. Because of this, I'd try especially hard to differentiate any offers from the typical format used by outsourcing spammers. Don't over-explain an offer. Keep it really brief.
If you need a portfolio, you could contact blockchain entrepreneurs with poor sites or logos and ask if they need help.
Otherwise and as well as, you'll want to build a personal brand in the space. Get on mailing lists or engage on social media. If you have an easy to remember business or personal name, avatar, etc then after a while people will become familiar with you. In any footer/profile/etc be sure to clearly outline what you do and what you want them to do. "I specialise in site design for blockchain entrepreneurs. Need free feedback on your project?" or similar. That at least gets you into a personal conversation.
Once you have projects under your belt, others in the space will notice footer credits or, if your work is especially good, hear about what you can do.