Water gets cold after ten minutes of use!
The black stuff should've been from the new Water Heater installation &amp; should've cleared right up. The Plumber likely had to shut-off the building's water &amp; the valve had some deposits scraped off from operation of the valve.

But, the hot water running out so quickly is pointing to a hot water line leak. Since it wasn't noticed by anyone you could have pipes under the concrete slab of the ground floor or basement.

3-ways to discover &amp; confirm a leak.

1 - Listen for hissing or slight high pitched rattling sounds or any unknown sound actually in the shower before turning anything on in the shower.

2 - Walk around in bare feet on the concrete slab to find a warm or hot spot.

3 - Turn off the Hot Water Valve at the Water Heater until your next shower &amp; then turn the hot water back on just before your shower. Just turn it on a little &amp; not wide open just 1/4 of the way.

If 1 &amp; 2 don't find anything initially, but 3 does improve the showering then you do have a leak &amp; go turn off the Hot Water after you're done using it again &amp; until your next need.

Turning off the Hot Water Valve stops the hot water from leaving the tank &amp; you get a full tank heating. Turning it back on just a little usually doesn't effect showering but limits how much hot water is lost through the leak. You can use the Water Heater this way indefinitely until the leak is found &amp; repaired.