Let me parade my ignorance in the hope that I can save others from my mistake. I tried over and over to link BlueLink with Google Assistant and over and over was told that my email and/or password was incorrect. I finally figured out what was happening. I was launching Google Home on my phone, clicking add, set up device, works with google and scrolled down to "BlueLink Smart Connect". It asks me to sign in to my account, and that's where it insists my credentials are wrong. Actually it was the wrong BlueLink. This is BlueLink for a thermostat. The right way is to click on the upper right corner, click services, scroll down to Explore and click it. In the search bar enter "Hyundai", click on "Hyundai Motor America" and that will bring up the options for the Hyundai Blue Link services. Oops. Once in the right app I was able to link accounts. That being said it still doesn't work very well, if at all. It's not just me, there are plenty of negative reviews.
So if you're up for some frustration you can follow the instructions above, or just stick with the Hyundai app. It may be slow but at least it works most of the time. Alexa works more reliably than Assistant, but you still need to first tell it to lock your car, wait for that action to complete, then tell it to start your car. Why in the world if you ask it to start your car can't it first send a lock signal followed by a start signal.