Shorthand for "UNIQUEIDENTIFIER" data type name?
The words guid and uid are coloured like any other object or possible column in a SQL statement in SSMS.
When typing out the columns in the following statement, each column is coloured blue, because it is a column from the tables I am referencing.
SELECT sao.name AS object_name,
       sao.type_desc AS object_type,
       sac.name AS column_name
FROM   sys.all_columns sac
       JOIN sys.all_objects sao
            ON  sac.object_id = sao.object_id
WHERE  sac.name LIKE 'guid'
       OR  sac.name LIKE 'uid' 

Coloured as per my SSMS:

The above statement queries all objects in the sys views all_objects and all_columns and displays the corresponding object type.

+--------------------------------------+----------------------------------+-------------+
|             object_name              |           object_type            | column_name |
+--------------------------------------+----------------------------------+-------------+
| sysguidrefs                          | SYSTEM_TABLE                     | guid        |
| sysobjects                           | VIEW                             | uid         |
| sysusers                             | VIEW                             | uid         |
| systypes                             | VIEW                             | uid         |
| sysprotects                          | VIEW                             | uid         |
| sysprocesses                         | VIEW                             | uid         |
| cryptographic_providers              | VIEW                             | guid        |
| syscacheobjects                      | VIEW                             | uid         |
| fn_trace_gettable                    | SQL_INLINE_TABLE_VALUED_FUNCTION | GUID        |
| dm_cryptographic_provider_properties | VIEW                             | guid        |
| dm_xe_packages                       | VIEW                             | guid        |
+--------------------------------------+----------------------------------+-------------+


As you can see, there are a couple of columns named either guid or uid in a SQL Server instance, that could possibly be referenced in a statement and this is why SSMS will colour code them blue.