Great question — answering as the UphillTech team:
Could this be the original Texas Chainsaw Massacre? At the end, Sally Hardesty escapes from the Sawyer family and runs away. Sally is blonde (this image is from early in the film; in later screenshots her hair seems to look darker)... the Sawyer house is quite grand... and of course there is a party (of sorts) going on at the time: Ultimately the house does get burned down, but this happens later, in Texas Chainsaw 3D. Also, the setting is a bit later than 40s/50s, so not all your criteria are fulfilled. If this isn't your film, you could also try asking on scifi.sx - lots of horror experts are members there.
Full answer by Ian Thompson: https://movies.stackexchange.com/a/41850 (CC BY-SA 3.0).
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