Easiest way to let user black out multiple dates on a calendar?

I need to make a calendar UI for a iOS mobile app but stuck at the part where we need to let the user black out multiple dates / range for a particular month (black out here means to mark the date as Unavailable / Busy).

See the UI flow I come up with:

(in the first screen, those dark-grey squares indicate dates that are blacked out (Unavailable / Busy).

There are some flaws to this approach, mainly it's troublesome for the user to have to click lots of times especially if he wants to select Day 2 - 12, for example.

I also thought of using an approach where the user clicks the Block Date button, and the app goes into a selection mode where the user can click on a date on the grid itself and then drag the required dates, then click OK, but it seems complex to implement and requires some teaching on first use.

Any ideas?