rear axle flexible brake hose questions 1972 dart

No it should not be like that. Hit it with some compressed air and blow that crud outta there; it may have been in the new line which is why I always blow them out,lol.
But to answer your question, with another question;
on the side with blockage, was there fluid in the old line? and wheel cylinder? If yes, then that points to the crud coming in the new line.
But if no, then yes; if it doesn't blow out, then drill it out or whatever it takes to get it out.
Be careful not to damage the tube-seat, cuz there is no fixing it, and the tiniest damage will make it leak, cuz the hydraulic pressure in there regularly runs 500 to 900psi.