I've never mapped for Source or Black Mesa, but I've worked with the original Half-Life content for longer than I care to admit.
The Black Mesa Research Facility has very distinct sectors that are very different from one another aesthetically. The "style" of each area really depends on the sector's function, its age and the people who work there.
The Anomalous Materials labs are all teal and poured concrete on the upper levels, relatively modern compared to the more industrial, heavy panelling of the lower levels. The office complex is very much stuck in the 1950s, all plaster walls and chequered floor tiles. The back areas and stairwells are less appealing to the eye, as they are not meant to be occupied for an extended period of time. The Questionable Ethics and Lambda Complex are far more modern and deal with a lot of high tech stuff. As such, they are clean and organised, but also full of exposed piping and wiring. The storage areas in We've Got Hostiles or Forget About Freeman are obviously basic concrete structures with wide open spaces for trucks and forklifts to move without hindrance.
It really does depend on what area you are building and what it was used for.
It's an underground complex, build into a decommissioned missile silo complex. Some areas are going to be refurbished, some completely disregarded and run down. Newer areas will likely be as cramped as possible, to save costs on the tunnelling and construction involved. Areas with a lot of employees during the day will have much easier access to cafeterias and restrooms, while the storage and freight areas will have smaller break rooms and very few places to do ones business.
To summarise, you just need to consider the following things and then design your map accordingly:
- What does this department do?
- What kind of people work here? How many?
- Is it new, old, refurbished or recently built?