To eat the donut and to have the donut 
I propose to prepare import script as follows:
/ip firewall address-list
:do { add address=X.X.X.X list=blackmail timeout=25h } on-error={set [find where address=X.X.X.X] timeout=25h}
:do { add address=Y.Y.Y.Y list=blackmail timeout=25h } on-error={set [find where address=Y.Y.Y.Y] timeout=25h}
I know, I know … it makes it bigger and import is slower but with one step we will have added new IPs, old ones included in the current update will stay in place with updated timeout and all adresses from old list not included in the new update will disappear soon naturally with their counting down timeout so the list will be self-cleaning.
We are ALL THE TIME protected.
EDIT:
Code should be:
/ip firewall address-list
:do { add address=X.X.X.X list=blackmail timeout=25h } on-error={set [find where address=X.X.X.X list=blackmail ] timeout=25h}
:do { add address=Y.Y.Y.Y list=blackmail timeout=25h } on-error={set [find where address=Y.Y.Y.Y list=blackmail ] timeout=25h}