The boy identified as Che Labastide-Wellington, from Harrow, was at a party in Belvedere Way, Kenton, on Saturday evening.
Police said there had been a fight with a group of youths who tried to gain entry,
Che was stabbed in a later scuffle in an alleyway at about 00:30 GMT.
He was pronounced dead about an hour later.
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A 16-year-old boy was also stabbed and taken to a north London hospital for treatment. His remains in a stable condition, said police.
They said Che had been invited to the party, which appeared to be "a well-run event with appropriate adults in attendance".
They believed he was not involved in an early altercation with a group of youths denied entry to the party.
But he appeared to have been targeted later, a spokesman said.
Det Chief Insp Chris Jones said he wanted to speak to anyone who was near the alleyway between Belvedere Way and Tylers Gate when Che was stabbed.
A man aged 21 has been arrested on suspicion of murder and taken to a west London police station for questioning.
Two boys aged 15 and 16 were also arrested on suspicion of murder and bailed until mid-December.
A third, aged 14, is being held on suspicion of possession of an offensive weapon.

James Alabi netted for the Blues early on after taking Elliott Durrell's cross with a deft touch before turning and rifling home.
John Askey's men equalised against the run of play when right-back Andy Halls made a forward run and played a neat one-two before placing past Chester goalkeeper Alex Lynch.
Jack Mackreth's superb cross then allowed Anthony Dudley to put Macc ahead with a firm header.
Alabi hit the post in the second half, but Kane Richards did eventually stab Chester level from another Durrell ball in.
Norburn, however, took on Rhys Browne's ball and struck in low from outside the area with five minutes remaining.
Match report supplied by the Press Association.
Match ends, Chester FC 2, Macclesfield Town 3.
Second Half ends, Chester FC 2, Macclesfield Town 3.
Substitution, Macclesfield Town. Chris Holroyd replaces Anthony Dudley.
Ollie Norburn (Macclesfield Town) is shown the yellow card.
Goal!  Chester FC 2, Macclesfield Town 3. Ollie Norburn (Macclesfield Town).
Goal!  Chester FC 2, Macclesfield Town 2. Kane Richards (Chester FC).
Danny Whitaker (Macclesfield Town) is shown the yellow card.
Elliott Durrell (Chester FC) is shown the yellow card.
Substitution, Chester FC. Theo Vassell replaces Danny O'Brien.
Substitution, Chester FC. Kane Richards replaces Blaine Hudson.
Second Half begins Chester FC 1, Macclesfield Town 2.
First Half ends, Chester FC 1, Macclesfield Town 2.
Goal!  Chester FC 1, Macclesfield Town 2. Anthony Dudley (Macclesfield Town).
Goal!  Chester FC 1, Macclesfield Town 1. Andy Halls (Macclesfield Town).
Goal!  Chester FC 1, Macclesfield Town 0. James Alabi (Chester FC).
First Half begins.
Lineups are announced and players are warming up.
A 17-year-old boy who was fatally stabbed in north-west London was at a 16th birthday party targeted by gatecrashers, police say.
[NEXT_CONCEPT]
Ollie Norburn grabbed a late winner as Macclesfield overcame Chester 3-2 at the Deva Stadium.