It's doubtful any definitive suggestion can be made with this query. Without more basic information and how iMessage is involved (is it the sender using an Android device and the recipient using an Apple device, or vice-versa???) So please provide some specifics, there are too many unanswered variables involved.
Apple refuses to allow any other platform to use or include support for its proprietary iMessage protocol, so all interactions between Android and Apple users always have to fall within the limited, and antiquated, restrictions involved with SMS/MMS protocols, so blocking on either platform may or may not be done solely within the text messaging apps in use (go into the app's Settings and clear the phone number in question out of the blocked numbers listing), or it might be tied to options settings within the messaging service(s) in use. (log into your account for your text messaging service or carrier account service using its web interface, and remove the phone number from there.)