Louisville's German numbers are perfectly valid, in both the inner city and the county as a whole. You will not find another single urban "Southern" county that has county-wide German ancestry in the same range as Jefferson County, KY, except in high-growth Florida counties where most residents are essentially relocated Midwesterners and Northeasterners.  I said county-wide, NOT just in reference to the inner cities which, as you stated, are often majority black.  Not in Dallas, Houston, San Antonio, Richmond, Nashville, Memphis, Charlotte, etc. etc. will you find similar German ancestry ranges - higher than 15 or so percent. 

German ancestry, county-wide
Jackson County, MO (23.3% black):  16.7% German

Jefferson County, KY (19% black):  18.2% German

Marion County, IN (24.2% black):  17.0% German

and, on the other hand:

Davidson County, TN:  7.8% German

Shelby County, TN:  5.3% German

Henrico County, VA:  9.1% German

Bexar County, TX (San Antonio):  8.6%

Harris County, TX:  7.2%

Dallas County, TX:  6.6%

Mecklenberg County, NC:  9.5%

Louisville's relatively low percentages of slaves weren't "unheard" of among slave states - especially for a border city - but they hardly support this notion of the city being "marginally Unionist."  You cannot compare Louisville's German populations to other Southern cities consistently across history...the fit just isn't there.  On the 1872 map, only the upper rim of the South (or lower rim of the Midwest, depending at how one looks at it) received higher influxes of German, and this it is shaded darker than portions further south.  The Germans are relevant here because they tended to despise slavery, given how they viewed it in relation to the serf system of their homeland.  The border states generally attracted fewer Germans because of the legality of slavery; when they did settle in these states, the Germans by and large chose cities like St. Louis, Louisville, Covington, etc. for economic opportunities and because of a general identification with the culture.

Yes, Louisville had a Confederate section of town.  Yes, Seattle, Washington has a Confederate memorial monument.  And?  There is no good reason to rewrite history to over-state the importance of a few dissidents.  Recognizing the city's Confederate influence would be one thing, but attempting to marginalize its importance/affinity for the Union in the war would be another entirely.