diff --git a/app/lib/activitypub/activity/block.rb b/app/lib/activitypub/activity/block.rb
index a17a2d50a..90477bf33 100644
--- a/app/lib/activitypub/activity/block.rb
+++ b/app/lib/activitypub/activity/block.rb
@@ -4,7 +4,12 @@ class ActivityPub::Activity::Block < ActivityPub::Activity
   def perform
     target_account = account_from_uri(object_uri)
 
-    return if target_account.nil? || !target_account.local? || @account.blocking?(target_account)
+    return if target_account.nil? || !target_account.local?
+
+    if @account.blocking?(target_account)
+      @account.block_relationships.find_by(target_account: target_account).update(uri: @json['id']) if @json['id'].present?
+      return
+    end
 
     UnfollowService.new.call(target_account, @account) if target_account.following?(@account)
 
diff --git a/app/lib/activitypub/activity/undo.rb b/app/lib/activitypub/activity/undo.rb
index 599823c6e..9eff1b71c 100644
--- a/app/lib/activitypub/activity/undo.rb
+++ b/app/lib/activitypub/activity/undo.rb
@@ -13,11 +13,62 @@ class ActivityPub::Activity::Undo < ActivityPub::Activity
       undo_like
     when 'Block'
       undo_block
+    when nil
+      handle_reference
     end
   end
 
   private
 
+  def handle_reference
+    # Some implementations do not inline the object, and as we don't have a
+    # global index, we have to guess what object it is.
+    return if object_uri.nil?
+
+    try_undo_announce || try_undo_accept || try_undo_follow || try_undo_like || try_undo_block || delete_later!(object_uri)
+  end
+
+  def try_undo_announce
+    status = Status.where.not(reblog_of_id: nil).find_by(uri: object_uri, account: @account)
+    if status.present?
+      RemoveStatusService.new.call(status)
+      true
+    else
+      false
+    end
+  end
+
+  def try_undo_accept
+    # We can't currently handle `Undo Accept` as we don't record `Accept`'s uri
+    false
+  end
+
+  def try_undo_follow
+    follow = @account.follow_requests.find_by(uri: object_uri) || @account.active_relationships.find_by(uri: object_uri)
+
+    if follow.present?
+      follow.destroy
+      true
+    else
+      false
+    end
+  end
+
+  def try_undo_like
+    # There is an index on accounts, but an account may have *many* favs, so this may be too costly
+    false
+  end
+
+  def try_undo_block
+    block = @account.block_relationships.find_by(uri: object_uri)
+    if block.present?
+      UnblockService.new.call(@account, block.target_account)
+      true
+    else
+      false
+    end
+  end
+
   def undo_announce
     return if object_uri.nil?
 
diff --git a/spec/lib/activitypub/activity/block_spec.rb b/spec/lib/activitypub/activity/block_spec.rb
index 94d37356d..42bdfdc81 100644
--- a/spec/lib/activitypub/activity/block_spec.rb
+++ b/spec/lib/activitypub/activity/block_spec.rb
@@ -28,6 +28,28 @@ RSpec.describe ActivityPub::Activity::Block do
     end
   end
 
+  context 'when the recipient is already blocked' do
+    before do
+      sender.block!(recipient, uri: 'old')
+    end
+
+    describe '#perform' do
+      subject { described_class.new(json, sender) }
+
+      before do
+        subject.perform
+      end
+
+      it 'creates a block from sender to recipient' do
+        expect(sender.blocking?(recipient)).to be true
+      end
+
+      it 'sets the uri to that of last received block activity' do
+        expect(sender.block_relationships.find_by(target_account: recipient).uri).to eq 'foo'
+      end
+    end
+  end
+
   context 'when the recipient follows the sender' do
     before do
       recipient.follow!(sender)
diff --git a/spec/lib/activitypub/activity/undo_spec.rb b/spec/lib/activitypub/activity/undo_spec.rb
index 9545e1f46..c0309e49d 100644
--- a/spec/lib/activitypub/activity/undo_spec.rb
+++ b/spec/lib/activitypub/activity/undo_spec.rb
@@ -50,6 +50,19 @@ RSpec.describe ActivityPub::Activity::Undo do
           expect(sender.reblogged?(status)).to be false
         end
       end
+
+      context 'with only object uri' do
+        let(:object_json) { 'bar' }
+
+        before do
+          Fabricate(:status, reblog: status, account: sender, uri: 'bar')
+        end
+
+        it 'deletes the reblog by uri' do
+          subject.perform
+          expect(sender.reblogged?(status)).to be false
+        end
+      end
     end
 
     context 'with Accept' do
@@ -91,13 +104,22 @@ RSpec.describe ActivityPub::Activity::Undo do
       end
 
       before do
-        sender.block!(recipient)
+        sender.block!(recipient, uri: 'bar')
       end
 
       it 'deletes block from sender to recipient' do
         subject.perform
         expect(sender.blocking?(recipient)).to be false
       end
+
+      context 'with only object uri' do
+        let(:object_json) { 'bar' }
+
+        it 'deletes block from sender to recipient' do
+          subject.perform
+          expect(sender.blocking?(recipient)).to be false
+        end
+      end
     end
 
     context 'with Follow' do
@@ -113,13 +135,22 @@ RSpec.describe ActivityPub::Activity::Undo do
       end
 
       before do
-        sender.follow!(recipient)
+        sender.follow!(recipient, uri: 'bar')
       end
 
       it 'deletes follow from sender to recipient' do
         subject.perform
         expect(sender.following?(recipient)).to be false
       end
+
+      context 'with only object uri' do
+        let(:object_json) { 'bar' }
+
+        it 'deletes follow from sender to recipient' do
+          subject.perform
+          expect(sender.following?(recipient)).to be false
+        end
+      end
     end
 
     context 'with Like' do