I want to do my due diligence and be as sure as I can be that ActBlue isn't legitimately just acting as the payment gateway, but it's really suspicious.
Their ToS is super sketch:
In the event that a campaign or committee (a) fails for 60 days to cash a check from ActBlue which includes your contribution (after ActBlue makes repeated attempts to work with the campaign to ensure all checks are cashed), or (b) affirmatively refuses a contribution earmarked through ActBlue, your contribution will be re-designated as a contribution to ActBlue.
So even if the money is going to "Black Lives Matter", BLM could easily just "affirmatively refuse the contribution" to funnel it right back to ActBlue.
If all they are is a payment gateway, I don't know why BLM wouldn't just use PayPal or Stripe or Auth.net
Why go through ActBlue?
Edit: This is what I found which contradicts the above
How do I receive my donations? If you’re a federal candidate, a c4 nonprofit, or a state-level candidate that did not have to set up a merchant account, you’ll receive a check each week containing funds from the previous week. We cut and mail checks every Monday, containing funds through Sunday at midnight.
If you’re a local candidate or state-level candidate that had to set up a merchant account, the money will be deposited directly into your bank account.https://support.actblue.com/campaigns/faq/donations-refunds-and-finances/
Okay it's a TOS from the DONATION link - https://secure.actblue.com/donate/ms_blm_homepage_2019
So there's no contradiction, they DO issue the check weekly but if you don't cash it they cancel it and keep the money. Nice scheme.