Hi,
You need to enhance the standard Web Dynpro component, then remove the Input Field UI for blood group( by using Remove Element) and create a new Drop Down UI.
Check the below docs for reference:
Enhance Standard WDA Component
Add Drop Down to Standard WDA Component
Or, You could create a Post Exit for WDDOMODIFY View method and create it via code dynamically.
For, your second query, again you have to enhance the standard component and create a post exit for onSelect Event handler method of First drop down and write the code to set the values to second drop down.
Hope this helps u,
Regards,
Kiran