Create another serializer say BLContainerSerializer and exclude fields there. Then use this in your BLcontainerMergedSerializer. Hope this helps.
class BLContainerSerializer(serializers.ModelSerializer):
class Meta:
model = Container
exclude = ('field1', )
class BLcontainerMergedSerializer(serializers.ModelSerializer):
container = BLContainerSerializer()
class Meta:
model = BLcontainer