Transparency of text over video is not working

Hi everyone, I am very new to openFrameworks and I am having lots of fun working with it so far!

I am trying to achieve semi-transparent text over video, which is drawn using ofVideoGrabber.

When I am using text with no transparency like this:

ofSetColor(51, 153, 0);
_goFont.drawString("GO!", ofGetWidth()/2, ofGetHeight()/2);

I am getting nice text:

View post on imgur.com

However when I want to add some transparency like this:

ofEnableAlphaBlending();
 ofSetColor(51, 153, 0, 127);
_goFont.drawString("GO!", ofGetWidth()/2, ofGetHeight()/2);
ofDisableAlphaBlending();

I am getting this black rectangle:

View post on imgur.com

Many thanks!