According to Developer, it's technically gray. With you having the 2-tone colors, it may look a little darker. I posted this in my post but can put it here too.

For now, you can change that Black/Grey Like "Thumbs up" icon by going into Developer > Data Items > System > Reactions

Edit the Like button there. Currently they have the color as grey:

Original: a:3:{s:4:"icon";s:9:"thumbs-up";s:5:"color";s:20:"sys-colored col-gray";s:6:"weight";s:1:"1";}

Change it to Blue

Original: a:3:{s:4:"icon";s:9:"thumbs-up";s:5:"color";s:20:"sys-colored col-blue";s:6:"weight";s:1:"1";}