Support blocking via query parameters as well and document the change.

lib/pleroma/web/api_spec/operations/domain_block_operation.ex

@@ -31,6 +31,7 @@ defmodule Pleroma.Web.ApiSpec.DomainBlockOperation do
     }
   end
 
+  # Supporting domain query parameter is deprecated in Mastodon API
   def create_operation do
     %Operation{
       tags: ["domain_blocks"],
@@ -45,11 +46,13 @@ defmodule Pleroma.Web.ApiSpec.DomainBlockOperation do
       """,
       operationId: "DomainBlockController.create",
       requestBody: domain_block_request(),
+      parameters: [Operation.parameter(:domain, :query, %Schema{type: :string}, "Domain name")],
       security: [%{"oAuth" => ["follow", "write:blocks"]}],
       responses: %{200 => empty_object_response()}
     }
   end
 
+  # Supporting domain query parameter is deprecated in Mastodon API
   def delete_operation do
     %Operation{
       tags: ["domain_blocks"],

lib/pleroma/web/mastodon_api/controllers/domain_block_controller.ex

@@ -32,6 +32,11 @@ defmodule Pleroma.Web.MastodonAPI.DomainBlockController do
     json(conn, %{})
   end
 
+  def create(%{assigns: %{user: blocker}} = conn, %{domain: domain}) do
+    User.block_domain(blocker, domain)
+    json(conn, %{})
+  end
+
   @doc "DELETE /api/v1/domain_blocks"
   def delete(%{assigns: %{user: blocker}, body_params: %{domain: domain}} = conn, _params) do
     User.unblock_domain(blocker, domain)