Safety and cabs (?) in Barcelona

The safety issue `in the past in Barrio Gothic is mostly that the narrow,, winding passageways can make unwary visitors — particularly intoxicated travelers—a potential target for pick pockets and thieves. Not knowing how your accommodation fits into the streets & alleyways of Barrio Gothic makes it impossible to know the risk level, but if you’re traveling with someone when returning from the concert and have your wits about you — you should be fine walking.
I was in Prague’s Old Town staying in an apartment some years back in an area of winding, narrow alleyways returning from a night out when I was accosted by a pair of thieves. The problem for the thieves was that I was totally sober, am very athletic, and I knew exactly how to fend them off.

If, after arriving at your hotel, you find it’s in a labyrinth of winding, narrow alleyways and are nervous about the situation— take a taxi.
These days, using Apple Pay and a single credit card makes having cash on your person unnecessary and this alone could be a deterrent to those trying to take advantage of tourists.
Enjoy Barcelona— it’s a wonderful city!

Also, wearing a money belt or neck wallet will help.

What is the concert venue?

Sam: Hotel at Placa Sant Josep / Placa dei Pi -- returning from concert at palau de la music orfeo catala -- thanks in advance for your input -- walking, taxi, uber?

Also, is there a preferred taxi app for Barcelona and/ or Madrid?

It looks like that is a 10-minute walk along major thoroughfares. You should be fine walking.