If you are relying on consent as the legal basis for this processing of personal data, then it MUST be opt-in. Pre-ticked checkboxes are not compliant.
If you are relying on a legitimate interest though, then an opt-out solution is OK. What falls under a legitimate interest can be fairly flexible, but you are required to show that this legitimate interest outweighs the “rights and freedoms” of the recipient.
It is not clear from your description which legal basis would be appropriate. This would depend on the contents of the emails. E.g. updates for new blog posts might be fine under a legitimate interest, but using the mailing list to disseminate marketing from third parties would likely require consent.
The ePrivacy directive has more specific rules on unsolicited communications, and allows you to send marketing emails about similar products or services to your own paying customers, if they can opt-out in the form where the email address was collected, e.g. by unticking a box, and if you include an unsubscribe link in every message:
Art 13(2): […] where a natural or legal person obtains from its customers their electronic contact details for electronic mail, in the context of the sale of a product or a service, in accordance with [the GDPR], the same natural or legal person may use these electronic contact details for direct marketing of its own similar products or services provided that customers clearly and distinctly are given the opportunity to object, free of charge and in an easy manner, to such use of electronic contact details at the time of their collection and on the occasion of each message in case the customer has not initially refused such use.
National laws might provide further details for this or other processing situations.