So i gave this another try.
First, i used rEFIt option as it was already installed. As earlier, when i restarted with usb i got three new options. 2 of them had EFI word in it, the third was boot from HD(it didn't say BIOS). So i used the third option and i got the black screen blinking hyphen for 20 mins and nothing happened.
Then i decided to remove rEFIT and installed rEFInd and on restarting with usb, i got an option which mentioned - Boot Fallback boot loader from USBname. I choose this option, intalled Win10, and on installing NVIDIA Win10 crashed again.
After browing some forums, it seems i had to partition the USB as MBR and not GPT. So i partioned the USB as MBR from mac and created win10 usb using Rufus which mentioned BIOS boot option formating.
Now i am getting the error which is preventing me to install-"Windows cannot be installed to this disk. The selected disk is of the GPT style.". On net, it say it is due to a mismatch between the way the HDD is formatted and the USB format style.
Should I reformat the HDD with MBR. Coz on GUID, with usb formatted in GUID gives me the black screen blinking hyphen problem.
Any help is highly appreciated.