It’s on their website
“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. Contributions to social welfare organizations which are similarly not cashed or affirmatively refused will be kept by ActBlue and used generally to support its social welfare activities. Contributions to charitable organizations which are not cashed or affirmatively refused will go to ActBlue Charities.”