[Baertram]

You could use a ZO_PreHook to the function CHAT_SYSTEM.OnChatEvent like this e.g.

function MyOnChatEvent(control, ...)
    --Setting enabled?
    secondsSinceMidnight = 0
    if settings.enableChatBlacklist and settings.chatKeyWords ~= nil and settings.chatKeyWords ~= "" then
        if settings.blacklistedTextToChat then
            --Get the current time
            secondsSinceMidnight = GetSecondsSinceMidnight()
        end
        --Filter the incoming chat message now
        local chatMessageWasBlacklisted = MyFunctionToCheckIfMessageWasBlacklistedWithBooleanReturnCode or false
        if chatMessageWasBlacklisted then
            --Abort the chat event function so no text is shown in the chat
            return true
        end
    end
    --Call the original chat event method now to show the text in the chat
    return false
end
ZO_PreHook(CHAT_SYSTEM, "OnChatEvent", MyOnChatEvent)

The secondsSinceMdinight is only there if you want to implement something like a "do not too often" or "wait 2 seconds before next blacklisted message check is done".