Hungarians, compared with Romanians I've encountered, (in this way similar to Poles) often have blue eyes but dark hair, brown usually, and of medium to stocky build, depending on where in the country they're from
Western Hungary is a good deal.... paler than Eastern Hungary. Budapest is very mixed. Hungarians tend to be.... stockier than Poles, which I largely put down to diet (Hungarian cuisine is delicious but extraordinarily fatty and Hungarian cakes and the like are significantly sweeter than Polish cakes) it all adds up.
There's plenty of Roma influence in Romania which might account for the swarthiness of many Romanians as well:-)
No. You don't understand Roman/Gypsy society. In Europe very few ever really integrate into the majority culture (outside of Spain and maybe Portugal). But many women work at some point in their lives as prostitutes so most of the mixing goes into (so to speak) the Roma. On the ground in Romania there's a higher percentage of mediterraneanish looks but a large majority could be plunked down in Poland and fit right in.
Romanian remains essentially a Romance tongue
Slavic influence on Romanian is almost entirley limited to vocabulary. Grammatically it's more a Balkan language, but that's a sprachbund (areal) classification and won't be found in the language family tree...
Hungarian is a weird case. It's unlike the Germanic, Romance and Slavic languages that surround it but also very unlike its closest genetic relations (according to a linguistics professor who studied them). Grammatically it definitely has some structures that seem loaned from German or Slavic (especially in terms of subordination - very weird).