Add transparent background to ffmpeg with text over it
Use the box options i.e. 

drawtext=enable='between(t,2.5,6.5)':fontsize=50:fontcolor=White:fontfile='ariblk.ttf':text='textline1':x=min(4*(tw\+10)-(abs(4-2*(t-2.5)))*(tw+10)-tw\,10):y=h-th-130:box=1:boxborderw=12:boxcolor=black@0.45


This adds a box with padding of 12 pixels around the text boundary, and the color is black with 45% opacity.

Using drawbox (add it before the drawtexts)

drawbox=y=ih-140:h=110:c=black@0.45:t=max:enable='between(t,2.5,6.5)'




Here's the entire vf expression:

-vf "drawbox=y=ih-140:h=110:c=black@0.45:t=max:enable='between(t,2.5,6.5)',drawtext=enable='between(t,2.5,6.5)':fontsize=50:fontcolor=White:fontfile='ariblk.ttf':text='textline1':x=min(4*(tw\+10)-(abs(4-2*(t-2.5)))*(tw+10)-tw\,10):y=h-th-130, drawtext=enable='between(t,2.5,6.5)':fontsize=50:fontcolor=White:fontfile='ariblk.ttf':text='textline2':x=min(4*(tw\+10)-(abs(4-2*(t-2.5)))*(tw+10)-tw\,10):y=h-th-75, drawtext=enable='between(t,2.5,6.5)':fontsize=50:fontcolor=White:fontfile='ariblk.ttf':text='textline3':x=min(4*(tw\+10)-(abs(4-2*(t-2.5)))*(tw+10)-tw\,10):y=h-th-20" -codec:a copy videoout.mp4