filling holes

Hi All Was not sure how to describe this issue but basically I removed the old rendering and discovered the wall is made up of several types of blocks and bricks as follows 1. very strong concrete blocks 2. very week brcks 3. very weak concrete blocks 'the darker coloured ones' the wall is...

Hi there Had a bit of an issue mounting some blinds and now I need to fill a hole (about 20mm x30mm, 20-30mm deep at max point) and then put in screws to remount blinds. Please see attached images for reference. Which type of filler would you recommend. Screws are 30mm long. Thanks